reserve A,X,X1,X2,Y,Y1,Y2 for set, a,b,c,d,x,y,z for object;
reserve P,P1,P2,Q,R,S for Relation;

theorem Th124:
  R"Y c= dom R
proof
  let x be object;
  assume x in R"Y;
  then ex y st [x,y] in R & y in Y by Def12;
  hence thesis by XTUPLE_0:def 12;
end;
