theorem Th3:
  [1,1] in [:Seg 1,Seg 3:] & [1,2] in [:Seg 1,Seg 3:] &
  [1,3] in [:Seg 1,Seg 3:]
  proof
    1 in Seg 3 & 2 in Seg 3 & 3 in Seg 3 & 1 in Seg 1 by FINSEQ_1:1;
    hence thesis by ZFMISC_1:87;
  end;
