Goal:
    G : t  HI : invariants  A : Prop  B : Prop  H : A <-> B  n : ~ B
  ============================
   (~ A)


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

No matching clauses for match.