sub make_iter {
   my ($base) = @_;

   my $num = Math::BigInt->new(1);
   my @digits = 1;  # -1 represents zero.
   my @powers = Math::BigInt->new(1);

   return sub {
      if ($digits[-1] > 0) {
         push @digits, -1;
         push @powers, $powers[-1] * $base;
      }

      for (0..$#digits) {
         $digits[$_] = -$digits[$_];
         $num += $digits[$_] * $powers[$_];
         return $num if $digits[$_] > 0;
      }
   };
}