theorem
  [:P,Q:] c= Indices A & (card P=0 iff card Q=0) implies
  Segm(A,P,Q)@ = Segm(A@,Q,P)
proof
  rng Sgm Q=Q & rng Sgm P=P by FINSEQ_1:def 14;
  hence thesis by Th18;
end;
