sub add_pair {
	my ($self, $key, $digest) = @_;
	return if $self->pair($key);
	$digest = Clusterize::Pattern->text2digest($digest)
		if ref $digest eq 'ARRAY';
	$self->pair($key, $digest);
	for (keys %{$digest}) {
		$self->add_cluster_pair($_, {key => $key, val => $digest->{$_}});
	}
}