theorem Thd:
  [dct.1, dct.2] in the S-T_Arcs of Dftn
  proof
A3: 1 mod 2 = 1 by NAT_D:14;
    len dct >= 3 by Def5;then
    1 + 1 < len dct by XXREAL_0:2;
    hence thesis by Def5, A3;
  end;
