Remark code_tail_bounds_1:
  forall fn ofs c,
  code_tail ofs fn c -> 0 <= ofs <= list_length_z fn.Remark code_tail_bounds_2:
  forall fn ofs i c,
  code_tail ofs fn (i :: c) -> 0 <= ofs < list_length_z fn.

Lemma code_tail_next: forall fn ofs i c, code_tail ofs fn (i :: c) -> code_tail (ofs + 1) fn c.