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

theorem
  X (/\) (X (/\) Y) = X (/\) Y
proof
  thus X (/\) (X (/\) Y) = X (/\) X (/\) Y by Th29
    .= X (/\) Y;
end;
