Goal:
  
  A : Type
  B : Type
  C : Type
  f : A → B → C
  a : A
  l0 : list A
  l0' : list A
  b : B
  l1 : list B
  l1' : list B
  IHl0 : ∀ (l0' : list A) (l1 l1' : list B),
           #|l0| = #|l1|
           → #|l0'| = #|l1'|
             → map2 f (l0 ++ l0') (l1 ++ l1') =
               map2 f l0 l1 ++ map2 f l0' l1'
  hl : #|l0| = #|l1|
  hl' : #|l0'| = #|l1'|
  ============================
   (#|l0| = #|l1|)


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]