module Substring_intf: Substring_intfval __pa_ounit_275876e34cf609db118f3d84b799a790 : stringmodule 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.