reserve X,X1,X2,Y,Y1,Y2 for set, p,x,x1,x2,y,y1,y2,z,z1,z2 for object;
reserve f,g,g1,g2,h for Function,
  R,S for Relation;

theorem Th74:
  f.:(f"Y) c= Y
proof
  let y be object;
  assume y in f.:(f"Y);
  then ex x being object st x in dom f & x in f"Y & y = f.x by Def6;
  hence thesis by Def7;
end;
