} else {      # remove
     if ( ! defined $fn ) { # remove if fileno undef'd
         defined($_) && $_ == $f and do { $vec->[FD_COUNT]--; $_ = undef; }
           for @{$vec}[FIRST_FD .. $#$vec];
         next; ###########################################################  ????
     }
     my $i = $fn + FIRST_FD;
     next unless defined $vec->[$i];
     $vec->[FD_COUNT]--;
     vec($bits, $fn, 1) = 0;
     $vec->[$i] = undef;
   }