public function validate_reply_to_id( $reply_id = 0 ) {

		// Bail if no reply
		if ( empty( $reply_id ) ) {
			return;
		}

		// Get posted reply to
		$new_reply_to = ! empty( $_POST['bbp_reply_to'] )
			? absint( $_POST['bbp_reply_to'] )
			: 0;

		// Bail if no reply to (assumes topic ID)
		if ( empty( $new_reply_to ) ) {
			return;
		}

		// Get current forum ID
		$forum_id = bbp_get_reply_forum_id( $reply_id );

		// Bail if not a group forum
		if ( ! bbp_is_forum_group_forum( $forum_id ) ) {
			return;
		}

		// Get current group ID
		$group_id = bp_get_current_group_id();

		// Bail if unknown group ID
		if ( empty( $group_id ) ) {
			return;
		}

		// Get current topic ID
		$topic_id = bbp_get_reply_topic_id( $reply_id );

		// Get topic reply IDs
		$reply_ids = bbp_get_public_child_ids( $topic_id, bbp_get_reply_post_type() );

		// Avoid recursion
		unset( $reply_ids[ $reply_id ] );

		// Bail if new reply parent ID is in this topic
		if ( in_array( $new_reply_to, $reply_ids, true ) ) {
			return;
		}

		// Add an error to prevent this reply from saving.
		bbp_add_error( 'bbp_reply_to_id', __( '<strong>ERROR</strong>: Reply To is invalid.', 'bbpress' ) );
	}