theorem Th18:
  for p being D-valued FinSequence holds p|(Seg a) is FinSequence of D
proof
  let p be D-valued FinSequence;
A1: p|(Seg a) is FinSequence by Th15;
  rng(p|(Seg a)) c= rng p & rng p c= D by RELAT_1:70,def 19;
  hence thesis by A1,Def4,XBOOLE_1:1;
end;
