diff --git a/src/libraries/turtle/operators.hpp b/src/libraries/turtle/operators.hpp
index b9193d6..562c306 100644
--- a/src/libraries/turtle/operators.hpp
+++ b/src/libraries/turtle/operators.hpp
@@ -98,9 +98,9 @@ namespace detail
 
     template< typename Functor >
     const constraint< detail::not_< Functor > >
-        operator!( const constraint< Functor >& ph )
+        operator!( const constraint< Functor >& c )
     {
-        return detail::not_< Functor >( ph.f_ );
+        return detail::not_< Functor >( c.f_ );
     }
 }