Goal:
  
  A : Type
  B : Type
  x : A
  xs : list A
  ih : ∀ l' : list B, map2 (λ (x : A) (_ : B), x) xs l' = firstn #|l'| xs
  y : B
  ys : list B
  ============================
   (locked false = true)


DEBUG:Going to execute:
Ltac.assumption
DEBUG LTAC Evaluated term: not_locked_false_eq_true