CHASE 0.1.1

• The slugs back end dropped the temporal operators F and G nested in a
  term, so G (req -> F grant) was checked as G (req -> grant) and
  is_realizable could answer unrealizable for a realizable contract (or
  the converse for assumptions). Terms outside GR(1) are now reported:
  is_realizable and synthesize return unknown with the term.