Goal:
  
  A : Type
  B : Type
  t' : A
  P : B → A → A → Type
  b : B
  bs : list B
  hd : A
  tl : list A
  p : P b hd t'
  e : #|bs| = #|tl|
  ============================
   ((hd = t') + (∑ i' : B, Some b = Some i' × P i' hd t'))%type


DEBUG:Going to execute:
match goal with
           | |- _ → _ => intro; t_tauto_intuit
           | |- _ => t_reduce; t_solver
           end