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

theorem Th93:
  X (/\) Y = X (\+\) (X (\) Y)
proof
A1: X (\) Y c= X by Th56;
  thus X (/\) Y = X (\) (X (\) Y) by Th68
    .= (X (\) (X (\) Y)) (\/) EmptyMS I by Th22,Th43
    .= X (\+\) (X (\) Y) by A1,Th52;
end;
