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

theorem
  X (/\) Y c= X (\/) Z
proof
  X (/\) Y c= X & X c= X (\/) Z by Th14,Th15;
  hence thesis by Th13;
end;
