{
	package Iterator::Filter;

	sub new
	{
		my $self = bless {}, shift;
		$self->{'iterator'} = shift;
		$self->{'condition'} = shift;
		$self->reset;
		$self
	}

	sub reset
	{
		my $self = shift;
		$self->{'iterator'}->reset;
		$self->_advance;
	}

	# post-condition: either 
	# a. iterator is not at end, and value is set; or
	# b. iterator is at end, and value is not set.
	sub _advance
	{
		my $self = shift;
		my @value;
		delete $self->{'value'};
		while ( !$self->{'iterator'}->is_exhausted )
		{
			@value = $self->{'iterator'}->value;
			if ( $self->{'condition'}->( @value ) )
			{
				$self->{'value'} = \@value;
				last;
			}
		}
	}

	sub value
	{
		my $self = shift;
        	$self->is_exhausted and die "past end";
		my $value = $self->{'value'};
		$self->_advance;
		@$value
	}

	sub is_exhausted
	{
		my $self = shift;
		!exists $self->{'value'}
	}
}