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:
<coq-core.plugins.ltac::contradiction@0> $1
Level 0: Backtrace:

No primitive equality found.