[boost] bjam question about "requires" negation

I want to run a test that should run without explicit operator conversion capability and fail to compile with explicit operation conversion capability, but I'm not sure how to negate the requires statement. I'm sure it's trivial but I found no examples. I tried a bunch of things but none worked. [ run implicit_conversion.cpp : : : [ requires NO cxx11_explicit_conversion_operators ] : implicit_conversion ] [ compile-fail implicit_conversion.cpp : [ requires cxx11_explicit_conversion_operators ] : explicit_conversion ] How do I properly implement the "NO" part? Thanks, Jim