theorem Th20:
  (for n holds seq.n=n) implies seq is divergent_to+infty
proof
  assume
A1: for n holds seq.n=n;
  let r;
  consider n such that
A2: r<n by SEQ_4:3;
  take n;
  let m;
  assume n<=m;
  then r<m by A2,XXREAL_0:2;
  hence thesis by A1;
end;
