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

theorem Th23:
  X c= Y implies X (/\) Y = X
proof
  assume
A1: X c= Y;
A2: X (/\) Y c= X by Th15;
  X c= X (/\) Y by A1,Th17;
 hence thesis by A2,Lm1;
end;
