package Settable;

sub new {
my($class,$set) = @_;
my $self = { Settable_set => $set };
return bless $self, $class;
}

sub set {
my ($self) = @_;
$self->{Settable_set} = 1;
}

package Collection; 

@ISA = qw(Settable);

sub new {
my($class, %items) = @_;
my $self = $class->SUPER::new();

self->{Collection_set} = { %items };
return $self;
}

sub list {
my($self) = @_;
print keys %{$self->{Collection_set}};
}