###
### powerset_recurse4
###

## Chapter 5 section 4.1.1

sub powerset_recurse ($) {
    my ( $set ) = @_;
    my $null = { };
    my $powerset  = { $null, $null };

    while (my ($key, $value) = each %$set) {

      my @newitems;

      while (my ($powerkey, $powervalue) = each %$powerset) {
          my %subset = ( );

          # Copy the old set to the subset.
          @subset{keys   %{ $powerset->{$powerkey} } } =
                  values %{ $powerset->{$powervalue} };

          # Add the new member to the subset.
          $subset{$key} = $value;

          # Prepare to add the new subset to the powerset.
          push @newitems, \%subset;
      }

      $powerset->{ $_ } = $_ for @newitems;

    }

    return $powerset;
}