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

theorem Th44:
  X c= EmptyMS I implies X = EmptyMS I
proof
  assume X c= EmptyMS I;
   then X c= EmptyMS I & EmptyMS I c= X by Th43;
  hence thesis by Lm1;
end;
