theorem Th12:
  p *(idseq n)=p & (idseq n)*p=p
proof
A1: Seg n = {} implies Seg n= {};
  p is Permutation of Seg n by Def12;
  then
A2: rng p= Seg n by FUNCT_2:def 3;
  p is Function of Seg n,Seg n by Def12;
  then dom p= Seg n by A1,FUNCT_2:def 1;
  hence thesis by A2,RELAT_1:52,54;
end;
