Goal:
    A : Type  H : ReflectEq A  a : A  l : list A
  ============================
   (let H := CoreTactics.block in
    let
    '(prefix, l1', l2') := split_prefix (a :: l) [] in
     a :: l = prefix ++ l1' ∧ [] = prefix ++ l2')


DEBUG:Going to execute:
fail 1