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

theorem Th75:
  (X (\/) Y) (\) Y = X (\) Y
proof
  thus (X (\/) Y) (\) Y = X (\) Y (\/) (Y (\) Y) by Th72
    .= X (\) Y (\/) EmptyMS I by Th52
    .= X (\) Y by Th22,Th43;
end;
