Goal:
    G : t  HI : invariants G  HG : acyclic_no_loop G  b : bool
  ============================
   (~ b -> ~~ b)


DEBUG:Going to execute:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end
Level 0: Backtrace:

No matching clauses for match.