theorem Th79:
  for p,q being FinSequence st q = p|Seg n holds len q <= len p
proof
  let p,q be FinSequence;
  assume q = p|Seg n;
  then
A1: dom q = dom p /\ Seg n by RELAT_1:61;
  Seg len q = dom q & Seg len p = dom p by Def3;
  hence thesis by Th5,A1,XBOOLE_1:17;
end;
