reserve i,j,e,u for object;
reserve I for set; 
reserve x,X,Y,Z,V for ManySortedSet of I;

theorem Th22:
  X c= Y implies X (\/) Y = Y
proof
  assume X c= Y;
  then
A1: X (\/) Y c= Y by Th16;
  Y c= X (\/) Y by Th14;
 hence thesis by A1,Lm1;
end;
