theorem Th10:
  [: NAT \ Segm m , NAT \ Segm n:] c= [:NAT,NAT:] \ [:Segm m,Segm n:]
  proof
A1: [:NAT,NAT:] \ [:Segm m,Segm n:] =
    [:NAT \ Segm m,NAT:] \/ [:NAT,NAT \ Segm n:] by ZFMISC_1:103;
    [: NAT \ Segm m , NAT \ Segm n:] =
    [:NAT \ Segm m,NAT:] \ [:NAT \ Segm m,Segm n:] by ZFMISC_1:102;
    hence thesis by A1,XBOOLE_1:10;
  end;
