Definition of the proto::when<> and proto::otherwise<> transforms.

namespace boost {
  namespace proto {
    template<typename Grammar, typename PrimitiveTransform = Grammar> 
      struct when;

    template<typename Grammar, typename Fun> struct when<Grammar, Fun *>;
    template<typename Grammar, typename R, typename... A> 
      struct when<Grammar, R(A...)>;

    template<typename Fun> struct otherwise;
  }
}