reserve i,j,k,l for natural Number;
reserve A for set, a,b,x,x1,x2,x3 for object;
reserve D,D9,E for non empty set;
reserve d,d1,d2,d3 for Element of D;
reserve d9,d19,d29,d39 for Element of D9;
reserve p,q,r for FinSequence;

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;
