Default "isinstance" type predicate functions / properties

[HolgerPeters]

As a library-user, I would like typecase to autogenerate predicate functions for variant types. I.e. when using

@variant
class Tree:
    Leaf = Tuple(object)
    Branch = Tuple(Object, Object)

I want to be able to call

>>> Leaf(4).is_leaf
True
>>> Leaf(4).is_branch
False

By autogenerating such lower-case method names, we would have to map the type names to such method names. Probably, enforcing that ADT type names are pep8 conforming would be good constraint to simplify method name generation.