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

theorem
  (id R)"X = X
proof
  rng id R = [#]R by RELAT_1:45;
  then
A1: (id R).:X = ((id R)/")"X by TOPS_2:54;
  (id R)/" = id R by Th2;
  hence thesis by A1,FUNCT_1:92;
end;
