I've hit something that I find weird in the behaviour of transforms. I don't know what is the intention. Basically, what seem to be happening is that if a transform doesn't cover all the cases but a pattern matches at some external level, then that is taken as a match. For instance, given the following transform: struct width_transform : proto::or_ < terminal_width<proto::terminal< number<_> > >, binary_width<proto::binary_expr<proto::tag::plus, width_transform, width_transform> > > {}; and evaluating the transform for proto::binary_expr<proto::tag::minus, something, something>, I get binary_width "called". If I add a pattern for proto::binary_expr<_, width_transform, width_transform> then that pattern is used. I would expect a failure at compile time, but in any case adding a wider pattern after a narrower pattern should never change whether the former matches or not. Unless I'm making something stupid, in which case just let me know, I'm used to it. Best regards, Maurizio