follows: In a first phase bind type variables to their instantiation for all producers. In the
second phase apply the substitution. This didn't work for consumers that are fused with multiple
producers, e.g:
  cons :: (a->b) (b->c) a -> c
  prod1 :: Int -> Int
  prod2 :: d->e
During producer-wise binding a and b were first bound to Int and Int. _Then_ b and c were bound
to d and e (b was overwritten)

Solution:
Apply the one substitution for each producer