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

theorem
  X (/\) Y = X (\+\) Y (\+\) (X (\/) Y)
proof
  thus X (/\) Y = X (\+\) (X (\) Y) by Th93
    .= X (\+\) (Y (\+\) (X (\/) Y)) by Th95
    .= X (\+\) Y (\+\) (X (\/) Y) by Th90;
end;
