teach the/2 about union types

[mndrix]

Most optionally typed languages support union types. Mavis should support them too.

The obvious syntax for unions should work fine:

the(atom;compound, Goal)

We luck out because the ; operator has lower priority than , does. This gives us a handy xfy list as the first argument:

?- write_canonical(the(color;flavor;odor, Foo)).
the(;(color,;(flavor,odor)),_)

It should be easy enough to combine that into a single freeze/2 call. Be careful not to introduce extra nondeterminism when checking the types.