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

theorem
  X c= Y & Z c= V implies X (\) V c= Y (\) Z
proof
  assume X c= Y & Z c= V;
  then X (\) V c= Y (\) V & Y (\) V c= Y (\) Z by Th53,Th54;
  hence thesis by Th13;
end;
