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
  ============================
   (x :: map2 (λ (x : A) (_ : B), x) xs ys = x :: firstn #|ys| xs)


DEBUG:Going to execute:
case not_locked_false_eq_true; Ltac.assumption
Level 1: Backtrace:

In environment
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
Unable to unify "x :: firstn #|ys| xs" with
 "x :: map2 (λ (x : A) (_ : B), x) xs ys".
Level 0: Backtrace:

In environment
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
Unable to unify "x :: firstn #|ys| xs" with
 "x :: map2 (λ (x : A) (_ : B), x) xs ys".
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.