theorem Th18:
  q = p|(Seg i) & len p <= i implies p = q
proof
  assume
A1: q = p|(Seg i);
  assume len p <= i;
  then Seg len p c= Seg i by FINSEQ_1:5;
  then dom p c= Seg i by FINSEQ_1:def 3;
  hence thesis by A1,RELAT_1:68;
end;
