diff --git a/src/libraries/turtle/constraints.hpp b/src/libraries/turtle/constraints.hpp
index e773924..2d7f81b 100644
--- a/src/libraries/turtle/constraints.hpp
+++ b/src/libraries/turtle/constraints.hpp
@@ -177,6 +177,12 @@ namespace detail
     {
         return detail::assign< T >( t );
     }
+
+    template< typename T >
+    constraint< T > call( T t )
+    {
+        return t;
+    }
 }
 
 #endif // #ifndef MOCK_CONSTRAINTS_HPP_INCLUDED
diff --git a/src/tests/turtle_test/constraints_test.cpp b/src/tests/turtle_test/constraints_test.cpp
index bae8ca8..40b4f76 100644
--- a/src/tests/turtle_test/constraints_test.cpp
+++ b/src/tests/turtle_test/constraints_test.cpp
@@ -7,6 +7,7 @@
 //
 
 #include <turtle/constraints.hpp>
+#include <turtle/operators.hpp>
 
 #include <boost/test/auto_unit_test.hpp>
 #define BOOST_LIB_NAME boost_unit_test_framework
@@ -165,6 +166,12 @@ namespace
     }
 }
 
+BOOST_AUTO_TEST_CASE( call )
+{
+    BOOST_CHECK( mock::call( &return_true ).f_() );
+    BOOST_CHECK( ! mock::call( &return_false ).f_() );
+}
+
 BOOST_AUTO_TEST_CASE( evaluate )
 {
     BOOST_CHECK( mock::evaluate.f_( &return_true ) );