reserve a,b,c,d,x,y,z for object, X,Y,Z for set;
reserve R,S,T for Relation;
reserve F,G for Function;

theorem
  (R |_2 X) |_2 Y = (R |_2 Y) |_2 X
proof
  thus (R |_2 X) |_2 Y = R |_2 (Y /\ X) by Th19
    .= (R |_2 Y) |_2 X by Th19;
end;
