module Substring_intf: Substring_intfmodule type S =sig..end
create ?pos ?len base creates a substring of the base sequence of
* length len starting at position pos, i.e.
*
* base.pos, base.pos + 1, ... base.pos + len - 1
*
* It is required that:
* 0 <= pos
* 0 <= len
* pos + len <= length base
*
* It does not copy the characters.