Goal:
  
  G : t
  HI : invariants
  A : Prop
  B : Prop
  b : B
  H0 : A -> B
  H1 : B -> A
  ============================
   A


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