Do not mutate underscore prefixed args or lvars

Changelog.md

@@ -1,5 +1,6 @@
 # v0.2.8 2012-12-29
 
+* [feature] Do not mutate argument or local variable names beginning with an underscore
 * [feature] Mutate unary calls ```coerce(object)``` => ```object```
 * [feature] Mutate method call receivers ```foo.bar``` => ```foo```
 

TODO

@@ -12,6 +12,11 @@ Mutations:
   * Mutate options on Regexp literals
   * Add mutations for dynamic regexp symbol and string literals
   * Mutate "def foo; bar; end" to "def foo; self; end"?
+  * Emit negative and positive mutations 
+   
+    Example of a negative mutation:
+      Mutations on local variables and arguments prefixed with an underscore would be emitted as
+      negative mutations that must be alive.
 
 Loader:
   * Make sure loader does not change visibility of injected mutants

lib/mutant/mutator/util/symbol.rb

@@ -16,7 +16,21 @@ module Mutant
         # @api private
         #
         def dispatch
-          emit_new { :"s#{Random.hex_string}" }
+          emit_new { :"s#{Random.hex_string}" } unless ignore?
+        end
+
+        # Test if symbol is ignored
+        #
+        # @return [true]
+        #   if symbol begins with an underscore
+        #
+        # @return [false]
+        #   otherwise
+        #
+        # @pai private
+        #
+        def ignore?
+          input.to_s[0] == '_'
         end
 
       end

spec/unit/mutant/mutator/node/define/mutation_spec.rb

@@ -51,6 +51,18 @@ describe Mutant::Mutator, 'define' do
     it_should_behave_like 'a mutator'
   end
 
+  context 'with arguments beginning with an underscore' do
+    let(:source) { 'def foo(_unused); end' }
+
+    let(:mutations) do
+      mutations = []
+      mutations << 'def foo(_unused); Object.new; end'
+      mutations << 'def foo; end'
+    end
+
+    it_should_behave_like 'a mutator'
+  end
+
   context 'default argument' do
     let(:source) { 'def foo(a = "literal"); end' }