theorem Th21:
  for f being non trivial FinSequence of TOP-REAL 2 holds f is_in_the_area_of f
proof
  let f be non trivial FinSequence of TOP-REAL 2;
  let n;
  assume
A1: n in dom f;
  len f >= 2 by NAT_D:60;
  then f/.n in L~f by A1,GOBOARD1:1;
  hence thesis by PSCOMP_1:24;
end;
