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) × P b hd hd')


DEBUG:Going to execute:
do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]