diff --git a/src/CitableBase.jl b/src/CitableBase.jl
index 79b164e..bf209a2 100644
--- a/src/CitableBase.jl
+++ b/src/CitableBase.jl
@@ -3,9 +3,10 @@ module CitableBase
 using Documenter, DocStringExtensions
 
 # Urn and its required functions:
-export Urn 
-export components, parts, validurn
+export Urn
 export dropversion, addversion
+# Concrete implementations:
+export components, parts
 
 # Citable and its required functions
 export Citable 
diff --git a/src/citable.jl b/src/citable.jl
index 42babb8..713cf55 100644
--- a/src/citable.jl
+++ b/src/citable.jl
@@ -1,21 +1,14 @@
-
-"Unique identifiers expressible in the syntax of the IETF's URN specification."
-abstract type Urn end 
-# Classes extending the URN abstractions MUST
-# have a member of type AbstractString named "urn":
-
-
-
 "A citable unit of any kind is identified by a URN and has a human-readable label."
 abstract type Citable end
 
 """Citable content should always be serializable.
 """
-function cex(data::T, delim="|") where {T <: Citable}
-    @warn("No cex function defined for ", typeof(data))
-end
+function cex end
 
-function objectid(data::T) where {T <: Citable}
-    @warn("No cex function defined for ", typeof(data))
-    nothing
-end
\ No newline at end of file
+"""Citable content has a label.
+"""
+function label end
+
+"""Citable content is identified by a URN.
+"""
+function urn end
\ No newline at end of file
diff --git a/src/urns.jl b/src/urns.jl
index 7251b5e..e017c98 100644
--- a/src/urns.jl
+++ b/src/urns.jl
@@ -20,10 +20,9 @@ $(SIGNATURES)
 Splits a URN's string representation into top-level components.
 """
 function components(u::Urn)
-    split(u.urn, ":")
+    split(string(u), ":")
 end
 
-
 """
 $(SIGNATURES)
 Splits a string on periods (seprator for parts within components of URNs).
diff --git a/test/runtests.jl b/test/runtests.jl
index 7402e10..15eb236 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -1,8 +1,6 @@
 using CitableBase
 using Test
 
-
 include("test_urnimpl.jl")
 include("test_urnmanipulation.jl")
 include("test_citeimpl.jl")
-include("test_cex.jl")
\ No newline at end of file
diff --git a/test/test_cex.jl b/test/test_cex.jl
deleted file mode 100644
index 4a21c2f..0000000
--- a/test/test_cex.jl
+++ /dev/null
@@ -1,12 +0,0 @@
-@testset "Test extending cex function for serialization" begin
-    struct PretendCitable <: Citable
-        col1
-        col2
-    end
-    function cex(pc::PretendCitable, delim="|")
-        join([pc.col1, pc.col2], delim)
-    end
-    pc = PretendCitable("x", "y")
-    @test cex(pc) == "x|y"
-    @test cex(pc, "#") == "x#y"
-end
\ No newline at end of file
diff --git a/test/test_citeimpl.jl b/test/test_citeimpl.jl
index ffd7438..1da46bf 100644
--- a/test/test_citeimpl.jl
+++ b/test/test_citeimpl.jl
@@ -20,4 +20,21 @@
     @test urn(c) == u
     @test label(c) == "label"
     @test cex(c) == "urn:fake:id.subid|label"
-end
\ No newline at end of file
+    
+end
+
+@testset "Test incomplete implementation of Citable" begin
+    struct PretendCitable <: Citable
+        col1
+        col2
+    end
+    function cex(pc::PretendCitable, delim="|")
+        join([pc.col1, pc.col2], delim)
+    end
+    pc = PretendCitable("x", "y")
+    @test cex(pc) == "x|y"
+    @test cex(pc, "#") == "x#y"
+    @test_throws MethodError urn(pc)
+    @test_throws MethodError label(pc)
+end
+