Goal:
  
  A : Type
  B : Type
  p : A → B → bool
  a : A
  l : list A
  b : B
  l' : list B
  IHl : ∀ l' : list B, forallb2 p l l' → #|l| = #|l'|
  ============================
   (p a b && forallb2 p l l' → S #|l| = S #|l'|)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2
Level 0: Backtrace:

Tactic failure: congruence failed.
Level 0: Backtrace:

Tactic failure: congruence failed.