Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  prefix : list A
  l1' : list A
  l2' : list A
  Heq : split_prefix l1 l2 = (prefix, l1', l2')
  Hind : ∀ (l1 := l1) (l2 := l2) (l : list A),
           is_prefix l l1
           → is_prefix l l2 → is_prefix l (fst (fst (split_prefix l1 l2)))
  Heq0 : (a1 == a2) = true
  l : list A
  Heqcall : (a1 :: prefix, l1', l2') = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    is_prefix l (a1 :: l1)
    → is_prefix l (a2 :: l2)
      → is_prefix l (fst (fst (split_prefix (a1 :: l1) (a2 :: l2)))))


DEBUG:Going to execute:
cbn[eq_rect_r eq_rect] in hyp