Goal:
    A : Type  H : ReflectEq A  a : A  l : list A  l0 : list A
  ============================
   (let H := CoreTactics.block in
    is_prefix l0 (a :: l)
    → is_prefix l0 [] → is_prefix l0 (fst (fst (split_prefix (a :: l) []))))


DEBUG:Going to execute:
fail 1