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

theorem Th30:
  X (/\) (X (\/) Y) = X
proof
  X c= X (\/) Y by Th14;
  then
A1: X c= X (/\) (X (\/) Y) by Th17;
 X (/\) (X (\/) Y) c= X by Th15;
  hence X (/\) (X (\/) Y) = X by A1,Lm1;
end;
