Goal:
  
  A : Type
  H : ReflectEq A
  y : A
  ys : list A
  _x_ : A
  _xs_ : list A
  tl : list A
  ============================
   (∃ l' : list A, ys ++ tl = ys ++ l')


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

Unable to find an instance for the variable x.
Level 0: Backtrace:

Unable to find an instance for the variable x.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.