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 : let l1 := l1 in
         let l2 := l2 in
         let
         '(prefix, l1', l2') := split_prefix l1 l2 in
          l1 = prefix ++ l1' ∧ l2 = prefix ++ l2'
  Heq0 : (a1 == a2) = true
  Heqcall : (a1 :: prefix, l1', l2') = split_prefix (a1 :: l1) (a2 :: l2)
  ============================
   (let H := CoreTactics.block in
    let
    '(prefix, l1', l2') := split_prefix (a1 :: l1) (a2 :: l2) in
     a1 :: l1 = prefix ++ l1' ∧ a2 :: l2 = prefix ++ l2')


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