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.