No variables available for parameterised types

[Chris Warburton]

For example, exploring a theory of lists, with `length`, `reverse`,
etc. we find things like:

  append [] [x] = [x]

We would expect to find the following, more general, theorem, since its
syntax tree is shallower:

  append [] y = y

The reason we don't is that we can't generate Arbitrary instances for
lists, and hence we don't get any list variables (like `y`). We can get
Arbitrary instances for non-parameterised types, like Nat, and hence
they get variables.

It looks like our parameterised types do have an instance, thanks to
TIP's translation, but it has an `Enumerable` constraint. Since we're
monomorphising everything to `Integer`, maybe we don't have an
`Enumerable` instance for `Integer`, and that's why we don't get an
Arbitrary instance?

Alternatively, it may be that ifcxt isn't choosing an instance because
it's not following the chain of constraints. Our patches to ifcxt should
have fixed this, but maybe we need to include something like
`mkIfCxtInstances ''Enumerable`?