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

theorem Th92:
  X (\/) Y = X (\+\) (Y (\) X)
proof
  thus X (\/) Y = Y (\/) (X (\/) X)
    .= X (\/) Y (\/) X by Th28
    .= (X (\) Y) (\/) Y (\/) X by Th67
    .= (X (\) Y) (\/) (X (\/) Y) by Th28
    .= (X (\) Y) (\/) (X (\/) (Y (\) X)) by Th67
    .= (X (\) Y) (\/) X (/\) X (\/) (Y (\) X) by Th28
    .= (X (\) (Y (\) X)) (\/) (Y (\) X) by Th64
    .= X (\+\) (Y (\) X) by Th78;
end;
