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

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