theorem
  i > 0 implies {[i,x]} is FinSubsequence
proof
  assume
A1: i > 0;
A2: dom {[i,x]} = {i} by RELAT_1:9;
  {i} c= Seg i
  proof
    let x be object;
    assume x in {i};
    then x = i by TARSKI:def 1;
    hence thesis by A1,Th3;
  end;
  hence thesis by A2,Def12;
end;
