Goal:
  
  A : Type
  B : Type
  P : A → B → B → Type
  b : A
  bs : list A
  hd : B
  hd' : B
  tl : list B
  p : P b hd hd'
  e : #|bs| = #|tl|
  ============================
   (∑ ni : nat, nth_error (b :: bs) ni = Some b)


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.