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

theorem
  (X (\/) Y) (/\) (X (\/) Z) = X implies Y (/\) Z c= X
proof
  assume (X (\/) Y) (/\) (X (\/) Z) = X;
  then X = X (\/) (Y (/\) Z) by Th33;
  hence thesis by Th14;
end;
