Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  x : A
  y : list B
  a : Forall2 R [x] y
  ============================
   (∃ y' : B, y = [y'] ∧ R x y')


DEBUG:Going to execute:
fun tac H =>
             DepElim.do_intros H; DepElim.generalize_by_eqs H; tac H