reserve x for set,
  i,j,k,n for Nat,
  K for Field;

theorem Th2:
  (Rev idseq 2).1 = 2 & (Rev idseq 2).2 = 1
proof
  reconsider f = idseq 2 as one-to-one FinSequence-like Function of Seg 2, Seg
  2;
  f.1 = (Rev f).len f by FINSEQ_5:62;
  then
A1: (idseq 2).1 = (Rev idseq 2).2 by CARD_1:def 7;
  f.(len f) = (Rev f).1 by FINSEQ_5:62;
  then
A2: f.2 = (Rev f).1 by CARD_1:def 7;
A3: dom Rev f = dom f by FINSEQ_5:57;
  then
A4: dom Rev f = Seg 2 by RELAT_1:45;
  then 1 in dom f & 2 in dom f by A3;
  hence thesis by A1,A2,A3,A4,FUNCT_1:18;
end;
