reserve i,j,e,u for object;
reserve I for set; 
reserve x,X,Y,Z,V for ManySortedSet of I;
reserve I for non empty set,
  x,X,Y for ManySortedSet of I;
reserve I for set,
  x,X,Y,Z for ManySortedSet of I;
reserve X for non-empty ManySortedSet of I;
reserve D for non empty set,
  n for Nat;
reserve X,Y for ManySortedSet of I;

theorem
 X c= Y & Y c= X implies X = Y by Lm1;
