Type improvement needs to correctly take into account the determiners and determinants

[fxdpntthm]

The try_type_improvement function assumes that the determinant is always the right most type argument to an open type

class F a b | a -> b
instance F Int Int

with
[W] F Int b -- works

T a b | b -> a
instance T Int int 

with [W] T a Int -- Fails

Expected
both T and F should work.