reserve S, R for 1-sorted,
  X for Subset of R,
  T for TopStruct,
  x for set;

theorem Th2:
  (id R)/" = id R
proof
  rng (id R) = [#]R by RELAT_1:45;
  then id R is onto;
  hence (id R)/" = (id the carrier of R)" by TOPS_2:def 4
    .= id R by FUNCT_1:45;
end;
