Goal:
  
  A : Type
  B : Type
  C : Type
  f : A → B → C
  l0' : list A
  b : B
  l1 : list B
  l1' : list B
  ============================
   (0 = #|b :: l1|
    → #|l0'| = #|l1'| → map2 f l0' ((b :: l1) ++ l1') = map2 f l0' l1')


DEBUG:Going to execute:
hnf