sub add
  {
    [...]
    $self->{_fhs}{"$fh"}{oldtie} = tied(*$fh) if defined(tied(*$fh));
    $self->{_handles}{"$fh"} = $fh;
    tie *$fh, "IO::Multiplex::Handle", $self, $fh;
    return $fh;
  }