@mbj yeah, having something responsible for testing it is a better idea. I suppose it's the only real solution for cases where you may be coercing to multiple types, like say with Boolean. You can't just do `value.kind_of?(TrueClass)` .. you also have to test if it is a `FalseClass` too.

I don't know if the coercer should be responsible for this, but I definitely think you're right in saying it shouldn't be the Attribute.