public function getItemTypes()
	{
		$usedTypes = array_reduce( $this->elements, function( $carry, $element ) {
			foreach ( $element as $key => $entry )
			{
				$type = $entry['taxonomy_element']['type'];
				if ( empty( $type ) ) continue;

				if ( ! isset( $carry[ $type ] ) )
				{
					$carry[ $type ] = 1;
				}
			}

			return $carry;
		}, array() );

		return array_keys( $usedTypes );
	}