Remove useless binary left negation

[Fix #1240]

* This orthogonal is not universal, especially in assignment cases.

lib/mutant/mutator/node/binary.rb

@@ -21,7 +21,6 @@ def dispatch
           emit_singletons
           emit_promotions
           emit_operator_mutations
-          emit_left_negation
           emit_left_mutations
           emit_right_mutations
         end
@@ -35,10 +34,6 @@ def emit_promotions
           emit(right)
         end
 
-        def emit_left_negation
-          emit(s(node.type, n_not(left), right))
-        end
-
       end # Binary
     end # Node
   end # Mutator

meta/and.rb

@@ -9,5 +9,4 @@
   mutation 'true or false'
   mutation 'false and false'
   mutation 'true and true'
-  mutation '!true and false'
 end

meta/or.rb

@@ -9,5 +9,17 @@
   mutation 'false or false'
   mutation 'true or true'
   mutation 'true and false'
-  mutation '!true or false'
+end
+
+Mutant::Meta::Example.add :or do
+  source 'a = true or false'
+
+  singleton_mutations
+  mutation 'false'
+  mutation 'a = true'
+  mutation 'a = false or false'
+  mutation 'a = true or true'
+  mutation 'a = true and false'
+  mutation 'nil or false'
+  mutation 'a__mutant__ = true or false'
 end