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

theorem
  X c= (Y (\/) Z) & X (/\) Z = EmptyMS I implies X c= Y
proof
  assume that
A1: X c= (Y (\/) Z) and
A2: X (/\) Z = EmptyMS I;
  X (/\) (Y (\/) Z)= X by A1,Th23;
  then Y (/\) X (\/) EmptyMS I = X by A2,Th32;
  then Y (/\) X = X by Th22,Th43;
  hence thesis by Th15;
end;
