Goal:
  
  A : Type
  H : ReflectEq A
  a1 : A
  l1 : list A
  a2 : A
  l2 : list A
  Heq : (a1 == a2) = false
  l3 : list A
  ============================
   (a2 :: l2 = l3
    → ([], a1 :: l1, a2 :: l2) = split_prefix (a1 :: l1) l3
      → let H := CoreTactics.block in
        let
        '(prefix, l1', l2') := split_prefix (a1 :: l1) l3 in
         a1 :: l1 = prefix ++ l1' ∧ l3 = prefix ++ l2')


DEBUG:Going to execute:
cbv beta