diff --git a/lib/lrama/command.rb b/lib/lrama/command.rb
index 0095c1a1..3ff39d57 100644
--- a/lib/lrama/command.rb
+++ b/lib/lrama/command.rb
@@ -19,7 +19,7 @@ def run(argv)
       text = options.y.read
       options.y.close if options.y != STDIN
       begin
-        grammar = Lrama::Parser.new(text, options.grammar_file, options.debug).parse
+        grammar = Lrama::Parser.new(text, options.grammar_file, options.debug, options.define).parse
         unless grammar.no_stdlib
           stdlib_grammar = Lrama::Parser.new(File.read(STDLIB_FILE_PATH), STDLIB_FILE_PATH, options.debug).parse
           grammar.insert_before_parameterizing_rules(stdlib_grammar.parameterizing_rules)
@@ -34,6 +34,7 @@ def run(argv)
       end
       states = Lrama::States.new(grammar, trace_state: (options.trace_opts[:automaton] || options.trace_opts[:closure]))
       states.compute
+      states.compute_ielr if grammar.ielr_defined?
       context = Lrama::Context.new(states)
 
       if options.report_file
diff --git a/lib/lrama/grammar.rb b/lib/lrama/grammar.rb
index eb567753..214ca1a3 100644
--- a/lib/lrama/grammar.rb
+++ b/lib/lrama/grammar.rb
@@ -28,14 +28,14 @@ class Grammar
     attr_reader :percent_codes, :eof_symbol, :error_symbol, :undef_symbol, :accept_symbol, :aux, :parameterizing_rule_resolver
     attr_accessor :union, :expect, :printers, :error_tokens, :lex_param, :parse_param, :initial_action,
                   :after_shift, :before_reduce, :after_reduce, :after_shift_error_token, :after_pop_stack,
-                  :symbols_resolver, :types, :rules, :rule_builders, :sym_to_rules, :no_stdlib, :locations
+                  :symbols_resolver, :types, :rules, :rule_builders, :sym_to_rules, :no_stdlib, :locations, :define
 
     def_delegators "@symbols_resolver", :symbols, :nterms, :terms, :add_nterm, :add_term, :find_term_by_s_value,
                                         :find_symbol_by_number!, :find_symbol_by_id!, :token_to_symbol,
                                         :find_symbol_by_s_value!, :fill_symbol_number, :fill_nterm_type,
                                         :fill_printer, :fill_destructor, :fill_error_token, :sort_by_number!
 
-    def initialize(rule_counter)
+    def initialize(rule_counter, define = {})
       @rule_counter = rule_counter
 
       # Code defined by "%code"
@@ -57,6 +57,7 @@ def initialize(rule_counter)
       @aux = Auxiliary.new
       @no_stdlib = false
       @locations = false
+      @define = define.map {|d| d.split('=') }.to_h
 
       append_special_symbols
     end
@@ -171,6 +172,10 @@ def find_rules_by_symbol(sym)
       @sym_to_rules[sym.number]
     end
 
+    def ielr_defined?
+      @define.key?('lr.type') && @define['lr.type'] == 'ielr'
+    end
+
     private
 
     def compute_nullable
diff --git a/lib/lrama/option_parser.rb b/lib/lrama/option_parser.rb
index 60da58d7..23988a5f 100644
--- a/lib/lrama/option_parser.rb
+++ b/lib/lrama/option_parser.rb
@@ -60,6 +60,7 @@ def parse_by_option_parser(argv)
         o.separator 'Tuning the Parser:'
         o.on('-S', '--skeleton=FILE', 'specify the skeleton to use') {|v| @options.skeleton = v }
         o.on('-t', '--debug', 'display debugging outputs of internal parser') {|v| @options.debug = true }
+        o.on('-D', '--define=NAME[=VALUE]', Array, "similar to '%define NAME VALUE'") {|v| @options.define = v }
         o.separator ''
         o.separator 'Output:'
         o.on('-H', '--header=[FILE]', 'also produce a header file named FILE') {|v| @options.header = true; @options.header_file = v }
diff --git a/lib/lrama/options.rb b/lib/lrama/options.rb
index ccd76803..08f75a77 100644
--- a/lib/lrama/options.rb
+++ b/lib/lrama/options.rb
@@ -7,10 +7,11 @@ class Options
                   :report_file, :outfile,
                   :error_recovery, :grammar_file,
                   :trace_opts, :report_opts,
-                  :diagnostic, :y, :debug
+                  :diagnostic, :y, :debug, :define
 
     def initialize
       @skeleton = "bison/yacc.c"
+      @define = {}
       @header = false
       @header_file = nil
       @report_file = nil
diff --git a/lib/lrama/parser.rb b/lib/lrama/parser.rb
index 237a34d6..177e784e 100644
--- a/lib/lrama/parser.rb
+++ b/lib/lrama/parser.rb
@@ -658,17 +658,18 @@ class Parser < Racc::Parser
 
 include Lrama::Report::Duration
 
-def initialize(text, path, debug = false)
+def initialize(text, path, debug = false, define = {})
   @grammar_file = Lrama::Lexer::GrammarFile.new(path, text)
   @yydebug = debug
   @rule_counter = Lrama::Grammar::Counter.new(0)
   @midrule_action_counter = Lrama::Grammar::Counter.new(1)
+  @define = define
 end
 
 def parse
   report_duration(:parse) do
     @lexer = Lrama::Lexer.new(@grammar_file)
-    @grammar = Lrama::Grammar.new(@rule_counter)
+    @grammar = Lrama::Grammar.new(@rule_counter, @define)
     @precedence_number = 0
     reset_precs
     do_parse
@@ -914,7 +915,7 @@ def raise_parse_error(error_message, location)
   2, 73, :_reduce_15,
   1, 60, :_reduce_none,
   2, 60, :_reduce_17,
-  3, 60, :_reduce_none,
+  3, 60, :_reduce_18,
   2, 60, :_reduce_none,
   2, 60, :_reduce_20,
   2, 60, :_reduce_21,
@@ -1328,7 +1329,12 @@ def _reduce_17(val, _values, result)
   end
 .,.,
 
-# reduce 18 omitted
+module_eval(<<'.,.,', 'parser.y', 27)
+  def _reduce_18(val, _values, result)
+     @grammar.define[val[1].s_value] = val[2]&.s_value
+    result
+  end
+.,.,
 
 # reduce 19 omitted
 
diff --git a/lib/lrama/state.rb b/lib/lrama/state.rb
index c2623746..3008786c 100644
--- a/lib/lrama/state.rb
+++ b/lib/lrama/state.rb
@@ -10,7 +10,7 @@ module Lrama
   class State
     attr_reader :id, :accessing_symbol, :kernels, :conflicts, :resolved_conflicts,
                 :default_reduction_rule, :closure, :items
-    attr_accessor :shifts, :reduces
+    attr_accessor :shifts, :reduces, :ielr_isocores, :lalr_isocore
 
     def initialize(id, accessing_symbol, kernels)
       @id = id
@@ -23,6 +23,12 @@ def initialize(id, accessing_symbol, kernels)
       @conflicts = []
       @resolved_conflicts = []
       @default_reduction_rule = nil
+      @predecessors = []
+      @lalr_isocore = self
+      @ielr_isocores = [self]
+      @internal_dependencies = {}
+      @successor_dependencies = {}
+      @always_follows = {}
     end
 
     def closure=(closure)
@@ -84,6 +90,18 @@ def transitions
       @transitions ||= shifts.map {|shift| [shift, @items_to_state[shift.next_items]] }
     end
 
+    def update_transition(shift, next_state)
+      set_items_to_state(shift.next_items, next_state)
+      next_state.append_predecessor(self)
+      clear_transitions_cache
+    end
+
+    def clear_transitions_cache
+      @nterm_transitions = nil
+      @term_transitions = nil
+      @transitions = nil
+    end
+
     def selected_term_transitions
       term_transitions.reject do |shift, next_state|
         shift.not_selected
@@ -142,5 +160,274 @@ def rr_conflicts
         conflict.type == :reduce_reduce
       end
     end
+
+    def propagate_lookaheads(next_state)
+      next_state.kernels.map {|item|
+        lookahead_sets =
+          if item.position == 1
+            goto_follow_set(item.lhs)
+          else
+            kernel = kernels.find {|k| k.predecessor_item_of?(item) }
+            item_lookahead_set[kernel]
+          end
+
+        [item, lookahead_sets & next_state.lookahead_set_filters[item]]
+      }.to_h
+    end
+
+    def lookaheads_recomputed
+      !@item_lookahead_set.nil?
+    end
+
+    def compatible_lookahead?(filtered_lookahead)
+      !lookaheads_recomputed ||
+        @lalr_isocore.annotation_list.all? {|token, actions|
+          a = dominant_contribution(token, actions, item_lookahead_set)
+          b = dominant_contribution(token, actions, filtered_lookahead)
+          a.nil? || b.nil? || a == b
+        }
+    end
+
+    def lookahead_set_filters
+      kernels.map {|kernel|
+        [kernel,
+         @lalr_isocore.annotation_list.select {|token, actions|
+           token.term? && actions.any? {|action, contributions|
+             !contributions.nil? && contributions.key?(kernel) && contributions[kernel]
+           }
+         }.map {|token, _| token }
+        ]
+      }.to_h
+    end
+
+    def dominant_contribution(token, actions, lookaheads)
+      a = actions.select {|action, contributions|
+        contributions.nil? || contributions.any? {|item, contributed| contributed && lookaheads[item].include?(token) }
+      }.map {|action, _| action }
+      return nil if a.empty?
+      a.reject {|action|
+        if action.is_a?(State::Shift)
+          action.not_selected
+        elsif action.is_a?(State::Reduce)
+          action.not_selected_symbols.include?(token)
+        end
+      }
+    end
+
+    def inadequacy_list
+      return @inadequacy_list if @inadequacy_list
+
+      shift_contributions = shifts.map {|shift|
+        [shift.next_sym, [shift]]
+      }.to_h
+      reduce_contributions = reduces.map {|reduce|
+        (reduce.look_ahead || []).map {|sym|
+          [sym, [reduce]]
+        }.to_h
+      }.reduce(Hash.new([])) {|hash, cont|
+        hash.merge(cont) {|_, a, b| a | b }
+      }
+
+      list = shift_contributions.merge(reduce_contributions) {|_, a, b| a | b }
+      @inadequacy_list = list.select {|token, actions| token.term? && actions.size > 1 }
+    end
+
+    def annotation_list
+      return @annotation_list if @annotation_list
+
+      @annotation_list = annotate_manifestation
+      @annotation_list = @items_to_state.values.map {|next_state| next_state.annotate_predecessor(self) }
+        .reduce(@annotation_list) {|result, annotations|
+          result.merge(annotations) {|_, actions_a, actions_b|
+            if actions_a.nil? || actions_b.nil?
+              actions_a || actions_b
+            else
+              actions_a.merge(actions_b) {|_, contributions_a, contributions_b|
+                if contributions_a.nil? || contributions_b.nil?
+                  next contributions_a || contributions_b
+                end
+
+                contributions_a.merge(contributions_b) {|_, contributed_a, contributed_b|
+                  contributed_a || contributed_b
+                }
+              }
+            end
+          }
+        }
+    end
+
+    def annotate_manifestation
+      inadequacy_list.transform_values {|actions|
+        actions.map {|action|
+          if action.is_a?(Shift)
+            [action, nil]
+          elsif action.is_a?(Reduce)
+            if action.rule.empty_rule?
+              [action, lhs_contributions(action.rule.lhs, inadequacy_list.key(actions))]
+            else
+              contributions = kernels.map {|kernel| [kernel, kernel.rule == action.rule && kernel.end_of_rule?] }.to_h
+              [action, contributions]
+            end
+          end
+        }.to_h
+      }
+    end
+
+    def annotate_predecessor(predecessor)
+      annotation_list.transform_values {|actions|
+        token = annotation_list.key(actions)
+        actions.transform_values {|inadequacy|
+          next nil if inadequacy.nil?
+          lhs_adequacy = kernels.any? {|kernel|
+            inadequacy[kernel] && kernel.position == 1 && predecessor.lhs_contributions(kernel.lhs, token).nil?
+          }
+          if lhs_adequacy
+            next nil
+          else
+            predecessor.kernels.map {|pred_k|
+              [pred_k, kernels.any? {|k|
+                inadequacy[k] && (
+                  pred_k.predecessor_item_of?(k) && predecessor.item_lookahead_set[pred_k].include?(token) ||
+                  k.position == 1 && predecessor.lhs_contributions(k.lhs, token)[pred_k]
+                )
+              }]
+            }.to_h
+          end
+        }
+      }
+    end
+
+    def lhs_contributions(sym, token)
+      shift, next_state = nterm_transitions.find {|sh, _| sh.next_sym == sym }
+      if always_follows(shift, next_state).include?(token)
+        nil
+      else
+        kernels.map {|kernel| [kernel, follow_kernel_items(shift, next_state, kernel) && item_lookahead_set[kernel].include?(token)] }.to_h
+      end
+    end
+
+    def follow_kernel_items(shift, next_state, kernel)
+      queue = [[self, shift, next_state]]
+      until queue.empty?
+        st, sh, next_st = queue.pop
+        return true if kernel.next_sym == sh.next_sym && kernel.symbols_after_transition.all?(&:nullable)
+        st.internal_dependencies(sh, next_st).each {|v| queue << v }
+      end
+      false
+    end
+
+    def item_lookahead_set
+      return @item_lookahead_set if @item_lookahead_set
+
+      kernels.map {|item|
+        value =
+          if item.lhs.accept_symbol?
+            []
+          elsif item.position > 1
+            prev_items = predecessors_with_item(item)
+            prev_items.map {|st, i| st.item_lookahead_set[i] }.reduce([]) {|acc, syms| acc |= syms }
+          elsif item.position == 1
+            prev_state = @predecessors.find {|p| p.shifts.any? {|shift| shift.next_sym == item.lhs } }
+            shift, next_state = prev_state.nterm_transitions.find {|shift, _| shift.next_sym == item.lhs }
+            prev_state.goto_follows(shift, next_state)
+          end
+        [item, value]
+      }.to_h
+    end
+
+    def item_lookahead_set=(k)
+      @item_lookahead_set = k
+    end
+
+    def predecessors_with_item(item)
+      result = []
+      @predecessors.each do |pre|
+        pre.items.each do |i|
+          result << [pre, i] if i.predecessor_item_of?(item)
+        end
+      end
+      result
+    end
+
+    def append_predecessor(prev_state)
+      @predecessors << prev_state
+      @predecessors.uniq!
+    end
+
+    def goto_follow_set(nterm_token)
+      return [] if nterm_token.accept_symbol?
+      shift, next_state = @lalr_isocore.nterm_transitions.find {|sh, _| sh.next_sym == nterm_token }
+
+      @kernels
+        .select {|kernel| follow_kernel_items(shift, next_state, kernel) }
+        .map {|kernel| item_lookahead_set[kernel] }
+        .reduce(always_follows(shift, next_state)) {|result, terms| result |= terms }
+    end
+
+    def goto_follows(shift, next_state)
+      queue = internal_dependencies(shift, next_state) + predecessor_dependencies(shift, next_state)
+      terms = always_follows(shift, next_state)
+      until queue.empty?
+        st, sh, next_st = queue.pop
+        terms |= st.always_follows(sh, next_st)
+        st.internal_dependencies(sh, next_st).each {|v| queue << v }
+        st.predecessor_dependencies(sh, next_st).each {|v| queue << v }
+      end
+      terms
+    end
+
+    def always_follows(shift, next_state)
+      return @always_follows[[shift, next_state]] if @always_follows[[shift, next_state]]
+
+      queue = internal_dependencies(shift, next_state) + successor_dependencies(shift, next_state)
+      terms = []
+      until queue.empty?
+        st, sh, next_st = queue.pop
+        terms |= next_st.term_transitions.map {|sh, _| sh.next_sym }
+        st.internal_dependencies(sh, next_st).each {|v| queue << v }
+        st.successor_dependencies(sh, next_st).each {|v| queue << v }
+      end
+      @always_follows[[shift, next_state]] = terms
+    end
+
+    def internal_dependencies(shift, next_state)
+      return @internal_dependencies[[shift, next_state]] if @internal_dependencies[[shift, next_state]]
+
+      syms = @items.select {|i|
+        i.next_sym == shift.next_sym && i.symbols_after_transition.all?(&:nullable) && i.position == 0
+      }.map(&:lhs).uniq
+      @internal_dependencies[[shift, next_state]] = nterm_transitions.select {|sh, _| syms.include?(sh.next_sym) }.map {|goto| [self, *goto] }
+    end
+
+    def successor_dependencies(shift, next_state)
+      return @successor_dependencies[[shift, next_state]] if @successor_dependencies[[shift, next_state]]
+
+      @successor_dependencies[[shift, next_state]] =
+        next_state.nterm_transitions
+        .select {|next_shift, _| next_shift.next_sym.nullable }
+        .map {|transition| [next_state, *transition] }
+    end
+
+    def predecessor_dependencies(shift, next_state)
+      state_items = []
+      @kernels.select {|kernel|
+        kernel.next_sym == shift.next_sym && kernel.symbols_after_transition.all?(&:nullable)
+      }.each do |item|
+        queue = predecessors_with_item(item)
+        until queue.empty?
+          st, i = queue.pop
+          if i.position == 0
+            state_items << [st, i]
+          else
+            st.predecessors_with_item(i).each {|v| queue << v }
+          end
+        end
+      end
+
+      state_items.map {|state, item|
+        sh, next_st = state.nterm_transitions.find {|shi, _| shi.next_sym == item.lhs }
+        [state, sh, next_st]
+      }
+    end
   end
 end
diff --git a/lib/lrama/states.rb b/lib/lrama/states.rb
index f56b0e9b..e7acd584 100644
--- a/lib/lrama/states.rb
+++ b/lib/lrama/states.rb
@@ -96,6 +96,20 @@ def compute
       report_duration(:compute_default_reduction) { compute_default_reduction }
     end
 
+    def compute_ielr
+      report_duration(:split_states) { split_states }
+      report_duration(:compute_direct_read_sets) { compute_direct_read_sets }
+      report_duration(:compute_reads_relation) { compute_reads_relation }
+      report_duration(:compute_read_sets) { compute_read_sets }
+      report_duration(:compute_includes_relation) { compute_includes_relation }
+      report_duration(:compute_lookback_relation) { compute_lookback_relation }
+      report_duration(:compute_follow_sets) { compute_follow_sets }
+      report_duration(:compute_look_ahead_sets) { compute_look_ahead_sets }
+      report_duration(:compute_conflicts) { compute_conflicts }
+
+      report_duration(:compute_default_reduction) { compute_default_reduction }
+    end
+
     def reporter
       StatesReporter.new(self)
     end
@@ -239,7 +253,7 @@ def enqueue_state(states, state)
       # Trace
       previous = state.kernels.first.previous_sym
       trace_state do |out|
-        out << sprintf("state_list_append (state = %d, symbol = %d (%s))",
+        out << sprintf("state_list_append (state = %d, symbol = %d (%s))\n",
           @states.count, previous.number, previous.display_name)
       end
 
@@ -269,7 +283,10 @@ def compute_lr0_states
         state.shifts.each do |shift|
           new_state, created = create_state(shift.next_sym, shift.next_items, states_created)
           state.set_items_to_state(shift.next_items, new_state)
-          enqueue_state(states, new_state) if created
+          if created
+            enqueue_state(states, new_state)
+            new_state.append_predecessor(state)
+          end
         end
       end
     end
@@ -528,5 +545,52 @@ def compute_default_reduction
         end.first
       end
     end
+
+    def split_states
+      transition_queue = []
+      @states.each do |state|
+        state.transitions.each do |shift, next_state|
+          compute_state(state, shift, next_state)
+        end
+      end
+    end
+
+    def merge_lookaheads(state, filtered_lookaheads)
+      return if state.kernels.all? {|item| (filtered_lookaheads[item] - state.item_lookahead_set[item]).empty? }
+
+      state.item_lookahead_set = state.item_lookahead_set.merge {|_, v1, v2| v1 | v2 }
+      state.transitions.each do |shift, next_state|
+        next if next_state.lookaheads_recomputed
+        compute_state(state, shift, next_state)
+      end
+    end
+
+    def compute_state(state, shift, next_state)
+      filtered_lookaheads = state.propagate_lookaheads(next_state)
+      s = next_state.ielr_isocores.find {|st| st.compatible_lookahead?(filtered_lookaheads) }
+
+      if s.nil?
+        s = next_state.ielr_isocores.last
+        new_state = State.new(@states.count, s.accessing_symbol, s.kernels)
+        new_state.closure = s.closure
+        new_state.compute_shifts_reduces
+        s.transitions.each do |sh, next_state|
+          new_state.set_items_to_state(sh.next_items, next_state)
+        end
+        @states << new_state
+        new_state.lalr_isocore = s
+        s.ielr_isocores << new_state
+        s.ielr_isocores.each do |st|
+          st.ielr_isocores = s.ielr_isocores
+        end
+        new_state.item_lookahead_set = filtered_lookaheads
+        state.update_transition(shift, new_state)
+      elsif(!s.lookaheads_recomputed)
+        s.item_lookahead_set = filtered_lookaheads
+      else
+        state.update_transition(shift, s)
+        merge_lookaheads(s, filtered_lookaheads)
+      end
+    end
   end
 end
diff --git a/lib/lrama/states/item.rb b/lib/lrama/states/item.rb
index 5074e943..e89cb969 100644
--- a/lib/lrama/states/item.rb
+++ b/lib/lrama/states/item.rb
@@ -64,6 +64,10 @@ def symbols_after_dot # steep:ignore
         rhs[position..-1]
       end
 
+      def symbols_after_transition
+        rhs[position+1..-1]
+      end
+
       def to_s
         "#{lhs.id.s_value}: #{display_name}"
       end
@@ -78,6 +82,10 @@ def display_rest
         r = symbols_after_dot.map(&:display_name).join(" ")
         ". #{r}  (rule #{rule_id})"
       end
+
+      def predecessor_item_of?(other_item)
+        rule == other_item.rule && position == other_item.position - 1
+      end
     end
   end
 end
diff --git a/parser.y b/parser.y
index 5d62e585..840797d0 100644
--- a/parser.y
+++ b/parser.y
@@ -25,7 +25,7 @@ rule
 
   bison_declaration: grammar_declaration
                    | "%expect" INTEGER { @grammar.expect = val[1] }
-                   | "%define" variable value
+                   | "%define" variable value { @grammar.define[val[1].s_value] = val[2]&.s_value }
                    | "%param" param+
                    | "%lex-param" param+
                        {
@@ -428,17 +428,18 @@ end
 
 include Lrama::Report::Duration
 
-def initialize(text, path, debug = false)
+def initialize(text, path, debug = false, define = {})
   @grammar_file = Lrama::Lexer::GrammarFile.new(path, text)
   @yydebug = debug
   @rule_counter = Lrama::Grammar::Counter.new(0)
   @midrule_action_counter = Lrama::Grammar::Counter.new(1)
+  @define = define
 end
 
 def parse
   report_duration(:parse) do
     @lexer = Lrama::Lexer.new(@grammar_file)
-    @grammar = Lrama::Grammar.new(@rule_counter)
+    @grammar = Lrama::Grammar.new(@rule_counter, @define)
     @precedence_number = 0
     reset_precs
     do_parse
diff --git a/sig/lrama/options.rbs b/sig/lrama/options.rbs
index d32dd675..2f8d5736 100644
--- a/sig/lrama/options.rbs
+++ b/sig/lrama/options.rbs
@@ -1,6 +1,7 @@
 module Lrama
   class Options
     attr_accessor skeleton: String
+    attr_accessor define: Hash[String, String]
     attr_accessor header: bool
     attr_accessor header_file: String?
     attr_accessor report_file: String?
diff --git a/spec/fixtures/integration/ielr.l b/spec/fixtures/integration/ielr.l
new file mode 100644
index 00000000..f4d782d6
--- /dev/null
+++ b/spec/fixtures/integration/ielr.l
@@ -0,0 +1,34 @@
+%option noinput nounput noyywrap never-interactive bison-bridge bison-locations
+
+%{
+
+#include <stdio.h>
+#include <stdlib.h>
+#include "bison-generated.h"
+#include "bison-generated-lexer.h"
+
+%}
+
+%%
+
+
+[abc] {
+    return yytext[0];
+}
+
+[\n|\r\n] {
+    return(YYEOF);
+}
+
+[[:space:]] {}
+
+<<EOF>> {
+    return(YYEOF);
+}
+
+. {
+    fprintf(stderr, "Illegal character '%s'\n", yytext);
+    return(YYEOF);
+}
+
+%%
diff --git a/spec/fixtures/integration/ielr.y b/spec/fixtures/integration/ielr.y
new file mode 100644
index 00000000..d8680a30
--- /dev/null
+++ b/spec/fixtures/integration/ielr.y
@@ -0,0 +1,62 @@
+%{
+#include <stdio.h>
+#include <stdlib.h>
+#include "y.tab.h"
+#define YYDEBUG 1
+static int yylex(YYSTYPE *val, YYLTYPE *loc);
+static int yyerror(YYLTYPE *loc, const char *str);
+%}
+
+%union {
+    int val;
+}
+
+%token a
+%token b
+%token c
+%define lr.type ielr
+
+%%
+S: a A B a
+ | b A B b
+A: a C D E
+B: c
+ | // empty
+C: D
+D: a
+E: a
+ | // empty
+
+%%
+
+static int yylex(YYSTYPE *yylval, YYLTYPE *loc) {
+    int c = getchar();
+    printf("%c\n", c);
+    int val;
+
+    switch (c) {
+    case ' ': case '\t':
+        return yylex(yylval, loc);
+
+    case 'a': case 'b': case 'c':
+        return c;
+
+    case '\n':
+        exit(0);
+
+    default:
+        fprintf(stderr, "unknown character: %c\n", c);
+        exit(1);
+    }
+}
+
+static int yyerror(YYLTYPE *loc, const char *str) {
+    fprintf(stderr, "parse error: %s\n", str);
+    return 0;
+}
+
+int main() {
+    printf("Enter the formula:\n");
+    yyparse();
+    return 0;
+}
diff --git a/spec/lrama/option_parser_spec.rb b/spec/lrama/option_parser_spec.rb
index 22560dd7..35b44940 100644
--- a/spec/lrama/option_parser_spec.rb
+++ b/spec/lrama/option_parser_spec.rb
@@ -49,6 +49,7 @@
           Tuning the Parser:
               -S, --skeleton=FILE              specify the skeleton to use
               -t, --debug                      display debugging outputs of internal parser
+              -D, --define=NAME[=VALUE]        similar to '%define NAME VALUE'
 
           Output:
               -H, --header=[FILE]              also produce a header file named FILE
diff --git a/spec/lrama/parser_spec.rb b/spec/lrama/parser_spec.rb
index 3581465c..14ef1728 100644
--- a/spec/lrama/parser_spec.rb
+++ b/spec/lrama/parser_spec.rb
@@ -59,6 +59,7 @@
       CODE
 
       expect(grammar.expect).to eq(0)
+      expect(grammar.define).to eq({'api.pure' => nil, 'parse.error' => 'verbose'})
       expect(grammar.printers).to eq([
         Printer.new(
           ident_or_tags: [T::Tag.new(s_value: "<int>")],
diff --git a/spec/lrama/states_spec.rb b/spec/lrama/states_spec.rb
index 953ac54c..c34c4749 100644
--- a/spec/lrama/states_spec.rb
+++ b/spec/lrama/states_spec.rb
@@ -1784,6 +1784,489 @@ class    go to state 5
             $default  reduce using rule 3 (expr)
 
 
+      STR
+    end
+  end
+
+  describe '#compute_ielr' do
+    it 'recompute states' do
+      path = "integration/ielr.y"
+      y = File.read(fixture_path(path))
+      grammar = Lrama::Parser.new(y, path).parse
+      grammar.prepare
+      grammar.validate!
+      states = Lrama::States.new(grammar)
+      states.compute
+      states.compute_ielr
+
+      io = StringIO.new
+      states.reporter.report(io, states: true)
+
+      expect(io.string).to eq(<<~STR)
+        State 14 conflicts: 2 shift/reduce
+
+
+        State 0
+
+            0 $accept: • S "end of file"
+
+            a  shift, and go to state 1
+            b  shift, and go to state 2
+
+            S  go to state 3
+
+
+        State 1
+
+            1 S: a • A B a
+
+            a  shift, and go to state 4
+
+            A  go to state 5
+
+
+        State 2
+
+            2 S: b • A B b
+
+            a  shift, and go to state 19
+
+            A  go to state 6
+
+
+        State 3
+
+            0 $accept: S • "end of file"
+
+            "end of file"  shift, and go to state 7
+
+
+        State 4
+
+            3 A: a • C D E
+
+            a  shift, and go to state 8
+
+            C  go to state 9
+            D  go to state 10
+
+
+        State 5
+
+            1 S: a A • B a
+
+            c  shift, and go to state 11
+
+            $default  reduce using rule 5 (B)
+
+            B  go to state 12
+
+
+        State 6
+
+            2 S: b A • B b
+
+            c  shift, and go to state 11
+
+            $default  reduce using rule 5 (B)
+
+            B  go to state 13
+
+
+        State 7
+
+            0 $accept: S "end of file" •
+
+            $default  accept
+
+
+        State 8
+
+            7 D: a •
+
+            $default  reduce using rule 7 (D)
+
+
+        State 9
+
+            3 A: a C • D E
+
+            a  shift, and go to state 8
+
+            D  go to state 14
+
+
+        State 10
+
+            6 C: D •
+
+            $default  reduce using rule 6 (C)
+
+
+        State 11
+
+            4 B: c •
+
+            $default  reduce using rule 4 (B)
+
+
+        State 12
+
+            1 S: a A B • a
+
+            a  shift, and go to state 15
+
+
+        State 13
+
+            2 S: b A B • b
+
+            b  shift, and go to state 16
+
+
+        State 14
+
+            3 A: a C D • E
+
+            a  shift, and go to state 17
+
+            a  reduce using rule 9 (E)
+            b  reduce using rule 9 (E)
+            c  reduce using rule 9 (E)
+
+            E  go to state 18
+
+
+        State 15
+
+            1 S: a A B a •
+
+            $default  reduce using rule 1 (S)
+
+
+        State 16
+
+            2 S: b A B b •
+
+            $default  reduce using rule 2 (S)
+
+
+        State 17
+
+            8 E: a •
+
+            $default  reduce using rule 8 (E)
+
+
+        State 18
+
+            3 A: a C D E •
+
+            $default  reduce using rule 3 (A)
+
+
+        State 19
+
+            3 A: a • C D E
+
+            a  shift, and go to state 8
+
+            C  go to state 20
+            D  go to state 10
+
+
+        State 20
+
+            3 A: a C • D E
+
+            a  shift, and go to state 8
+
+            D  go to state 21
+
+
+        State 21
+
+            3 A: a C D • E
+
+            a  shift, and go to state 17
+
+            $default  reduce using rule 9 (E)
+
+            E  go to state 18
+
+
+      STR
+    end
+
+    it 'recompute states' do
+      y = <<~INPUT
+        %{
+        // Prologue
+        %}
+
+        %token <val> NUM
+        %token tEQ "=="
+        %type <val> expr
+
+        %nonassoc tEQ
+
+        %%
+
+        program : /* empty */
+             | expr { printf("=> %d", $1); }
+             ;
+
+        expr : NUM
+             | expr tEQ expr { $$ = $1; }
+             ;
+
+        %%
+      INPUT
+      grammar = Lrama::Parser.new(y, "states/ielr_nonassoc.y").parse
+      grammar.prepare
+      grammar.validate!
+      states = Lrama::States.new(grammar)
+      states.compute
+      states.compute_ielr
+
+      io = StringIO.new
+      states.reporter.report(io, states: true)
+
+      expect(io.string).to eq(<<~STR)
+        State 0
+
+            0 $accept: • program "end of file"
+
+            NUM  shift, and go to state 1
+
+            $default  reduce using rule 1 (program)
+
+            program  go to state 2
+            expr     go to state 3
+
+
+        State 1
+
+            3 expr: NUM •
+
+            $default  reduce using rule 3 (expr)
+
+
+        State 2
+
+            0 $accept: program • "end of file"
+
+            "end of file"  shift, and go to state 4
+
+
+        State 3
+
+            2 program: expr •
+            4 expr: expr • "==" expr
+
+            "=="  shift, and go to state 5
+
+            $default  reduce using rule 2 (program)
+
+
+        State 4
+
+            0 $accept: program "end of file" •
+
+            $default  accept
+
+
+        State 5
+
+            4 expr: expr "==" • expr
+
+            NUM  shift, and go to state 1
+
+            expr  go to state 6
+
+
+        State 6
+
+            4 expr: expr • "==" expr
+            4     | expr "==" expr •
+
+            "=="  error (nonassociative)
+            "=="  error (nonassociative)
+
+            $default  reduce using rule 4 (expr)
+
+
+      STR
+    end
+
+    it 'recompute states' do
+      y = <<~INPUT
+        %{
+        // Prologue
+        %}
+
+        %token NUM
+
+        %nonassoc  tCMP
+        %left '>'
+        %left '+'
+
+        %%
+
+        program : arg
+                ;
+
+        arg : arg '+' arg
+            | rel_expr    %prec tCMP
+            | NUM
+            ;
+
+        relop : '>'
+              ;
+
+        rel_expr : arg relop arg   %prec '>'
+                 ;
+
+        %%
+      INPUT
+      grammar = Lrama::Parser.new(y, "states/ielr_prec.y").parse
+      grammar.prepare
+      grammar.validate!
+      states = Lrama::States.new(grammar)
+      states.compute
+      states.compute_ielr
+
+      io = StringIO.new
+      states.reporter.report(io, states: true)
+
+      expect(io.string).to eq(<<~STR)
+        State 0
+
+            0 $accept: • program "end of file"
+
+            NUM  shift, and go to state 1
+
+            program   go to state 2
+            arg       go to state 3
+            rel_expr  go to state 4
+
+
+        State 1
+
+            4 arg: NUM •
+
+            $default  reduce using rule 4 (arg)
+
+
+        State 2
+
+            0 $accept: program • "end of file"
+
+            "end of file"  shift, and go to state 5
+
+
+        State 3
+
+            1 program: arg •
+            2 arg: arg • '+' arg
+            6 rel_expr: arg • relop arg
+
+            '>'  shift, and go to state 6
+            '+'  shift, and go to state 7
+
+            $default  reduce using rule 1 (program)
+
+            relop  go to state 8
+
+
+        State 4
+
+            3 arg: rel_expr •
+
+            $default  reduce using rule 3 (arg)
+
+
+        State 5
+
+            0 $accept: program "end of file" •
+
+            $default  accept
+
+
+        State 6
+
+            5 relop: '>' •
+
+            $default  reduce using rule 5 (relop)
+
+
+        State 7
+
+            2 arg: arg '+' • arg
+
+            NUM  shift, and go to state 1
+
+            arg       go to state 9
+            rel_expr  go to state 4
+
+
+        State 8
+
+            6 rel_expr: arg relop • arg
+
+            NUM  shift, and go to state 1
+
+            arg       go to state 10
+            rel_expr  go to state 4
+
+
+        State 9
+
+            2 arg: arg • '+' arg
+            2    | arg '+' arg •
+            6 rel_expr: arg • relop arg
+
+            $default  reduce using rule 2 (arg)
+
+            relop  go to state 8
+
+
+        State 10
+
+            2 arg: arg • '+' arg
+            6 rel_expr: arg • relop arg
+            6         | arg relop arg •
+
+            '+'  shift, and go to state 7
+
+            $default  reduce using rule 6 (rel_expr)
+
+            relop  go to state 11
+
+
+        State 11
+
+            6 rel_expr: arg relop • arg
+
+            NUM  shift, and go to state 1
+
+            arg       go to state 12
+            rel_expr  go to state 4
+
+
+        State 12
+
+            2 arg: arg • '+' arg
+            6 rel_expr: arg • relop arg
+            6         | arg relop arg •
+
+            '+'  shift, and go to state 7
+
+            $default  reduce using rule 6 (rel_expr)
+
+            relop  go to state 11
+
+
       STR
     end
   end