Goal:
  
  A : Type
  v : A
  a : A
  l : list A
  H : (∃ n : nat, hd_error (skipn n l) = Some v) ↔ In v l
  ============================
   (locked false = true)


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