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 (\) (Y (\/) Z) (\/) X (/\) Y (/\) Z
proof
  thus X (\) (Y (\+\) Z) = X (\) ((Y (\/) Z) (\) Y (/\) Z) by Th87
    .= X (\) (Y (\/) Z) (\/) X (/\) (Y (/\) Z) by Th64
    .= X (\) (Y (\/) Z) (\/) X (/\) Y (/\) Z by Th29;
end;
