reserve A,B for Ordinal,
  K,M,N for Cardinal,
  x,x1,x2,y,y1,y2,z,u for object,X,Y,Z,X1,X2, Y1,Y2 for set,
  f,g for Function;
reserve m,n for Nat;
reserve x1,x2,x3,x4,x5,x6,x7,x8 for object;

theorem
  Z <> {} & Z is finite & (for X,Y st X in Z & Y in Z holds X c= Y or Y
  c= X) implies union Z in Z by Lm6;
