Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → C → Type
  g : A → B → C
  x : A
  xs : list A
  y : B
  ys : list B
  eq : #|xs| = #|ys|
  r : R x (g x y)
  z : All2 R xs (map2 g xs ys)
  ============================
   (#|xs| = #|ys|)


DEBUG:Going to execute:
trivial; hnf; intros