- Ambiguous signatures are not detected properly in the presence of multiparameter
  type classes (implementation of fixv is wrong).
  Testcases:
  tests/multi-param-typeclasses/well-formed/should_fail/ambiguous2.phc
  tests/multi-param-typeclasses/well-formed/should_fail/ambiguous3.phc

- Unification seems to have problems with parameterized ATs
  Testcase:
  tests/examples/stack.phc