diff --git a/README b/README
index f8250b7..4e16675 100644
--- a/README
+++ b/README
@@ -41,12 +41,14 @@ Already implemented algorithms
 
 
 Coming up
-=========        
+=========
 
-  * Does polygon contain given point?
+  * Is polygon convex?  
 
   * Do polygons intersect?
 
+  * Does polygon contain given point?
+
   * Rectangular bounds of polygon
 
   * Area of polygon
diff --git a/TODO b/TODO
new file mode 100644
index 0000000..38a2614
--- /dev/null
+++ b/TODO
@@ -0,0 +1,35 @@
+== Auxiliary features
+* Methods requests tagging
+
+If algorithm requires some state or property of object, it should be tagged.
+Requirements should be checked before each method call (of course, info should
+be cached due to computational complexity of algorithms).
+Example:
+
+class Polygon
+
+  def convex?
+    ...
+  end
+
+  def self_intersecting?
+    ...
+  end
+
+  must_be :convex
+  must_not_be :self_intersecting
+  def contains_point?(point)
+    ... # code, which assumes, that polygon is convex and not self_intersecting
+  end
+
+== Algorithms
+
+* Polygon convexity
+* Polygons intersection
+* Polygon contains given point?
+* Rectangular bounds of polygon
+* Area of polygon
+* Does circle contain given point?
+* Do circles intersect?
+* Area of circle
+* Making a ruby gem
\ No newline at end of file
diff --git a/lib/vector.rb b/lib/vector.rb
index f35382a..a15c93f 100644
--- a/lib/vector.rb
+++ b/lib/vector.rb
@@ -32,7 +32,7 @@ def +(vector)
     end
 
     def -(vector)
-      self + vector * -1
+      self + (-1) * vector
     end
 
     def *(scalar)      
@@ -43,7 +43,7 @@ def coerce(scalar)
       if scalar.is_a?(Numeric)
         [self, scalar]
       else
-        raise ArgumentError, "Cannot coerce #{scalar.inspect}"
+        raise ArgumentError, "Vector: cannot coerce #{scalar.inspect}"
       end             
     end
   end