, etc.

			# Possible improvement: print the warning, followed by the contents
			# of the  tag.  This way no part of the article will be eaten
			# even temporarily.

			$this->referenceStack->pushInvalidRef();
			return $this->errorReporter->halfParsed( 'cite_error_included_ref' );
		}

		if ( is_string( $name ) || is_string( $text ) ) {
			# We don't care about the content: if the name exists, the ref
			# is presumptively valid.  Either it stores a new ref, or re-
			# fers to an existing one.  If it refers to a nonexistent ref,
			# we'll figure that out later.  Likewise it's definitely valid
			# if there's any content, regardless of name.

			$result = $this->referenceStack->pushRef(
				$text, $name, $group, $follow, $argv, $dir, $parser->getStripState() );
			if ( $result === null ) {
				return '';
			} else {
				[ $key, $count, $label, $subkey ] = $result;
				return $this->linkRef( $group, $key, $count, $label, $subkey );
			}
		}

		# Not clear how we could get here, but something is probably
		# wrong with the types.  Let's fail fast.
		throw new Exception( 'Invalid $text and/or $name: ' . serialize( [ $text, $name ] ) );
	}

	/**
	 * Deals with references defined in the reference section
	 * 
	 *  BAR 
	 * 
	 *
	 * @param string|false|null $name
	 * @param string|null $text Content from the  tag
	 * @param string $group
	 * @param bool $isSectionPreview
	 */
	private function inReferencesGuardedRef( $name, $text, $group, $isSectionPreview ) {
		if ( $group !== $this->inReferencesGroup ) {
			#  and  have conflicting group attributes.
			$this->mReferencesErrors[] = $this->errorReporter->halfParsed(
				'cite_error_references_group_mismatch',
				Sanitizer::safeEncodeAttribute( $group )
			);
		} elseif ( $name === null || $name === '' ) {
			#  calls inside  must be named
			$this->mReferencesErrors[] = $this->errorReporter->halfParsed(
				'cite_error_references_no_key'
			);
		} elseif ( $text === '' ) {
			#  called in  has no content.
			$this->mReferencesErrors[] = $this->errorReporter->halfParsed(
				'cite_error_empty_references_define',
				Sanitizer::safeEncodeAttribute( $name )
			);
		} elseif ( !$this->referenceStack->hasGroup( $group ) && !$isSectionPreview ) {
			# Called with group attribute not defined in text.
			$this->mReferencesErrors[] = $this->errorReporter->halfParsed(
				'cite_error_references_missing_group',
				Sanitizer::safeEncodeAttribute( $group )
			);
		} else {
			$groupRefs = $this->referenceStack->getGroupRefs( $group );
			if ( !isset( $groupRefs[$name] ) && !$isSectionPreview ) {
				# Called with name attribute not defined in text.
				$this->mReferencesErrors[] = $this->errorReporter->halfParsed(
					'cite_error_references_missing_key',
					Sanitizer::safeEncodeAttribute( $name )
				);
			} elseif ( isset( $groupRefs[$name]['text'] ) && $groupRefs[$name]['text'] !== $text ) {
				// two refs with same key and different content
				// add error message to the original ref
				$text = $groupRefs[$name]['text'] . ' ' .
					$this->errorReporter->plain( 'cite_error_references_duplicate_key', $name );
				$this->referenceStack->setRefText( $group, $name, $text );
			} else {
				$this->referenceStack->setRefText( $group, $name, $text );
			}
		}
	}

	/**
	 * Parse the arguments to the  tag
	 *
	 *  "name" : Name for reusing the reference.
	 *  "group" : Group to which it belongs. Needs to be passed to  too.
	 *  "follow" : If the current reference is the continuation of a named reference.
	 *  "dir" : set direction of text (ltr/rtl)
	 *  "extends": Points to a named reference which serves as the context for this reference.
	 *
	 * @param string[] $argv The argument vector
	 * @return (string|false|null)[] An array with exactly four elements, where each is a string on
	 *  valid input, false on invalid input, or null on no input.
	 * @return-taint tainted
	 */
	private function refArg( array $argv ) {
		global $wgCiteBookReferencing;

		$group = null;
		$name = null;
		$follow = null;
		$dir = null;
		$extends = null;

		if ( isset( $argv['dir'] ) ) {
			$dir = trim( $argv['dir'] );
			unset( $argv['dir'] );
		}

		if ( $argv === [] ) {
			// No more attributes.
			return [ null, null, null, $dir, null ];
		}

		if ( isset( $argv['follow'] ) &&
			( isset( $argv['name'] ) || isset( $argv[self::BOOK_REF_ATTRIBUTE] ) )
		) {
			return [ false, false, false, null, false ];
		}

		if ( isset( $argv['name'] ) ) {
			// Key given.
			$name = trim( $argv['name'] );
			unset( $argv['name'] );
		}
		if ( isset( $argv['follow'] ) ) {
			// Follow given.
			$follow = trim( $argv['follow'] );
			unset( $argv['follow'] );
		}
		if ( isset( $argv['group'] ) ) {
			// Group given.
			$group = $argv['group'];
			unset( $argv['group'] );
		}
		if ( $wgCiteBookReferencing && isset( $argv[self::BOOK_REF_ATTRIBUTE] ) ) {
			$extends = trim( $argv[self::BOOK_REF_ATTRIBUTE] );
			unset( $argv[self::BOOK_REF_ATTRIBUTE] );
		}

		if ( $argv !== [] ) {
			// Unexpected invalid attribute.
			return [ false, false, false, false, false ];
		}

		return [ $name, $group, $follow, $dir, $extends ];
	}

	/**
	 * Callback function for 
	 *
	 * @param string|null $text Raw content of the  tag.
	 * @param string[] $argv Arguments
	 * @param Parser $parser
	 *
	 * @return string|false False in case a  tag is not allowed in the current context
	 */
	public function references( $text, array $argv, Parser $parser ) {
		if ( $this->mInCite || $this->inReferencesGroup !== null ) {
			return false;
		}

		$this->rememberParser( $parser );
		$ret = $this->guardedReferences( $text, $argv, $parser );
		$this->inReferencesGroup = null;

		return $ret;
	}

	/**
	 * Must only be called from references(). Use that to prevent recursion.
	 *
	 * @param string|null $text Raw content of the  tag.
	 * @param string[] $argv
	 * @param Parser $parser
	 *
	 * @return string
	 */
	private function guardedReferences(
		$text,
		array $argv,
		Parser $parser
	) {
		global $wgCiteResponsiveReferences;

		$group = $argv['group'] ?? self::DEFAULT_GROUP;
		unset( $argv['group'] );
		$this->inReferencesGroup = $group;

		if ( strval( $text ) !== '' ) {
			# Detect whether we were sent already rendered s.
			# Mostly a side effect of using #tag to call references.
			# The following assumes that the parsed s sent within
			# the  block were the most recent calls to
			# .  This assumption is true for all known use cases,
			# but not strictly enforced by the parser.  It is possible
			# that some unusual combination of #tag,  and
			# conditional parser functions could be created that would
			# lead to malformed references here.
			$count = substr_count( $text, Parser::MARKER_PREFIX . "-ref-" );

			# Undo effects of calling  while unaware of containing 
			$redoStack = $this->referenceStack->rollbackRefs( $count );

			# Rerun  call now that mInReferences is set.
			foreach ( $redoStack as $call ) {
				[ $ref_argv, $ref_text ] = $call;
				$this->guardedRef( $ref_text, $ref_argv, $parser );
			}

			# Parse $text to process any unparsed  tags.
			$parser->recursiveTagParse( $text );
		}

		if ( isset( $argv['responsive'] ) ) {
			$responsive = $argv['responsive'] !== '0';
			unset( $argv['responsive'] );
		} else {
			$responsive = $wgCiteResponsiveReferences;
		}

		// There are remaining parameters we don't recognise
		if ( $argv ) {
			return $this->errorReporter->halfParsed( 'cite_error_references_invalid_parameters' );
		}

		$s = $this->referencesFormat( $group, $responsive );

		# Append errors generated while processing 
		if ( $this->mReferencesErrors ) {
			$s .= "\n" . implode( "
\n", $this->mReferencesErrors );
			$this->mReferencesErrors = [];
		}
		return $s;
	}

	/**
	 * Make output to be returned from the references() function.
	 *
	 * If called outside of references(), caller is responsible for ensuring
	 * `mInReferences` is enabled before the call and disabled after call.
	 *
	 * @param string $group
	 * @param bool $responsive
	 * @return string HTML ready for output
	 */
	private function referencesFormat( $group, $responsive ) {
		if ( !$this->referenceStack->hasGroup( $group ) ) {
			return '';
		}

		// Add new lines between the list items (ref entries) to avoid confusing tidy (T15073).
		// Note: This builds a string of wikitext, not html.
		$parserInput = "\n";
		$groupRefs = $this->referenceStack->getGroupRefs( $group );
		foreach ( $groupRefs as $key => $value ) {
			$parserInput .= $this->referencesFormatEntry( $key, $value ) . "\n";
		}
		$parserInput = Html::rawElement( 'ol', [ 'class' => [ 'references' ] ], $parserInput );

		// Live hack: parse() adds two newlines on WM, can't reproduce it locally -ævar
		$ret = rtrim( $this->mParser->recursiveTagParse( $parserInput ), "\n" );

		if ( $responsive ) {
			// Use a DIV wrap because column-count on a list directly is broken in Chrome.
			// See https://bugs.chromium.org/p/chromium/issues/detail?id=498730.
			$wrapClasses = [ 'mw-references-wrap' ];
			if ( count( $groupRefs ) > 10 ) {
				$wrapClasses[] = 'mw-references-columns';
			}
			$ret = Html::rawElement( 'div', [ 'class' => $wrapClasses ], $ret );
		}

		if ( !$this->mParser->getOptions()->getIsPreview() ) {
			// save references data for later use by LinksUpdate hooks
			$this->saveReferencesData( $this->mParser->getOutput(), $group );
		}

		// done, clean up so we can reuse the group
		$this->referenceStack->deleteGroup( $group );

		return $ret;
	}

	/**
	 * Format a single entry for the referencesFormat() function
	 *
	 * @param string|int $key The key of the reference
	 * @param array $val A single reference as documented at {@see ReferenceStack::$refs}
	 * @return string Wikitext, wrapped in a single 
 element
	 */
	private function referencesFormatEntry( $key, array $val ) {
		$text = $this->referenceText( $key, $val['text'] );
		$error = '';
		$extraAttributes = '';

		if ( isset( $val['dir'] ) ) {
			$dir = strtolower( $val['dir'] );
			if ( in_array( $dir, [ 'ltr', 'rtl' ] ) ) {
				$extraAttributes = Html::expandAttributes( [ 'class' => 'mw-cite-dir-' . $dir ] );
			} else {
				$error .= $this->errorReporter->plain( 'cite_error_ref_invalid_dir', $val['dir'] ) . "\n";
			}
		}

		// Fallback for a broken, and therefore unprocessed follow="…". Note this returns a 
, not
		// an 
 as expected!
		if ( isset( $val['follow'] ) ) {
			return wfMessage(
				'cite_references_no_link',
				$this->normalizeKey(
					self::getReferencesKey( $val['follow'] )
				),
				$text
			)->inContentLanguage()->plain();
		}

		// This counts the number of reuses. 0 means the reference appears only 1 time.
		if ( isset( $val['count'] ) && $val['count'] < 1 ) {
			// Anonymous, auto-numbered references can't be reused and get marked with a -1.
			if ( $val['count'] < 0 ) {
				$id = $val['key'];
				$backlinkId = $this->refKey( $val['key'] );
			} else {
				$id = $key . '-' . $val['key'];
				$backlinkId = $this->refKey( $key, $val['key'] . '-' . $val['count'] );
			}
			return wfMessage(
				'cite_references_link_one',
				$this->normalizeKey( self::getReferencesKey( $id ) ),
				$this->normalizeKey( $backlinkId ),
				$text . $error,
				$extraAttributes
			)->inContentLanguage()->plain();
		}

		// Named references with >1 occurrences
		$backlinks = [];
		// There is no count in case of a section preview
		for ( $i = 0; $i <= ( $val['count'] ?? -1 ); $i++ ) {
			$backlinks[] = wfMessage(
				'cite_references_link_many_format',
				$this->normalizeKey(
					$this->refKey( $key, $val['key'] . '-' . $i )
				),
				$this->referencesFormatEntryNumericBacklinkLabel(
					$val['number'],
					$i,
					$val['count']
				),
				$this->referencesFormatEntryAlternateBacklinkLabel( $i )
			)->inContentLanguage()->plain();
		}
		return wfMessage(
			'cite_references_link_many',
			$this->normalizeKey(
				self::getReferencesKey( $key . '-' . ( $val['key'] ?? '' ) )
			),
			$this->listToText( $backlinks ),
			$text . $error,
			$extraAttributes
		)->inContentLanguage()->plain();
	}

	/**
	 * Returns formatted reference text
	 * @param string|int $key
	 * @param string|null $text
	 * @return string
	 */
	private function referenceText( $key, $text ) {
		if ( trim( $text ) === '' ) {
			if ( $this->mParser->getOptions()->getIsSectionPreview() ) {
				return $this->errorReporter->plain( 'cite_warning_sectionpreview_no_text', $key );
			}
			return $this->errorReporter->plain( 'cite_error_references_no_text', $key );
		}
		return '' . rtrim( $text, "\n" ) . "\n";
	}

	/**
	 * Generate a numeric backlink given a base number and an
	 * offset, e.g. $base = 1, $offset = 2; = 1.2
	 * Since bug #5525, it correctly does 1.9 -> 1.10 as well as 1.099 -> 1.100
	 *
	 * @param int $base
	 * @param int $offset
	 * @param int $max Maximum value expected.
	 * @return string
	 */
	private function referencesFormatEntryNumericBacklinkLabel( $base, $offset, $max ) {
		$scope = strlen( $max );
		$ret = MediaWikiServices::getInstance()->getContentLanguage()->formatNum(
			sprintf( "%s.%0{$scope}s", $base, $offset )
		);
		return $ret;
	}

	/**
	 * Generate a custom format backlink given an offset, e.g.
	 * $offset = 2; = c if $this->mBacklinkLabels = [ 'a',
	 * 'b', 'c', ...]. Return an error if the offset > the # of
	 * array items
	 *
	 * @param int $offset
	 *
	 * @return string
	 */
	private function referencesFormatEntryAlternateBacklinkLabel( $offset ) {
		if ( !isset( $this->mBacklinkLabels ) ) {
			$this->genBacklinkLabels();
		}
		return $this->mBacklinkLabels[$offset]
			?? $this->errorReporter->plain( 'cite_error_references_no_backlink_label', null );
	}

	/**
	 * Generate a custom format link for a group given an offset, e.g.
	 * the second  is b if $this->mLinkLabels["foo"] =
	 * [ 'a', 'b', 'c', ...].
	 * Return an error if the offset > the # of array items
	 *
	 * @param int $offset
	 * @param string $group The group name
	 * @param string $label The text to use if there's no message for them.
	 *
	 * @return string
	 */
	private function getLinkLabel( $offset, $group, $label ) {
		$message = "cite_link_label_group-$group";
		if ( !isset( $this->mLinkLabels[$group] ) ) {
			$this->genLinkLabels( $group, $message );
		}
		if ( $this->mLinkLabels[$group] === false ) {
			// Use normal representation, ie. "$group 1", "$group 2"...
			return $label;
		}

		return $this->mLinkLabels[$group][$offset - 1]
			?? $this->errorReporter->plain( 'cite_error_no_link_label_group', [ $group, $message ] );
	}

	/**
	 * Return an id for use in wikitext output based on a key and
	 * optionally the number of it, used in , not 
	 * (since otherwise it would link to itself)
	 *
	 * @param string $key
	 * @param int|null $num The number of the key
	 * @return string A key for use in wikitext
	 */
	private function refKey( $key, $num = null ) {
		$prefix = wfMessage( 'cite_reference_link_prefix' )->inContentLanguage()->text();
		$suffix = wfMessage( 'cite_reference_link_suffix' )->inContentLanguage()->text();
		if ( $num !== null ) {
			$key = wfMessage( 'cite_reference_link_key_with_num', $key, $num )
				->inContentLanguage()->plain();
		}

		return "$prefix$key$suffix";
	}

	/**
	 * Return an id for use in wikitext output based on a key and
	 * optionally the number of it, used in , not 
	 * (since otherwise it would link to itself)
	 *
	 * @param string $key
	 * @return string A key for use in wikitext
	 */
	public static function getReferencesKey( $key ) {
		$prefix = wfMessage( 'cite_references_link_prefix' )->inContentLanguage()->text();
		$suffix = wfMessage( 'cite_references_link_suffix' )->inContentLanguage()->text();

		return "$prefix$key$suffix";
	}

	/**
	 * Generate a link ( element from a key
	 * and return XHTML ready for output
	 *
	 * @suppress SecurityCheck-DoubleEscaped
	 * @param string $group
	 * @param string $key The key for the link
	 * @param int|null $count The index of the key, used for distinguishing
	 *                   multiple occurrences of the same key
	 * @param int $label The label to use for the link, I want to
	 *                   use the same label for all occurrences of
	 *                   the same named reference.
	 * @param string|null $subkey
	 *
	 * @return string
	 */
	private function linkRef( $group, $key, $count, $label, $subkey ) {
		$contLang = MediaWikiServices::getInstance()->getContentLanguage();

		return $this->mParser->recursiveTagParse(
				wfMessage(
					'cite_reference_link',
					$this->normalizeKey(
						$this->refKey( $key, $count )
					),
					$this->normalizeKey(
						self::getReferencesKey( $key . $subkey )
					),
					Sanitizer::safeEncodeAttribute(
						$this->getLinkLabel( $label, $group,
							( ( $group === self::DEFAULT_GROUP ) ? '' : "$group " ) . $contLang->formatNum( $label ) )
					)
				)->inContentLanguage()->plain()
			);
	}

	/**
	 * Normalizes and sanitizes a reference key
	 *
	 * @param string $key
	 * @return string
	 */
	private function normalizeKey( $key ) {
		$ret = Sanitizer::escapeIdForAttribute( $key );
		$ret = preg_replace( '/__+/', '_', $ret );
		$ret = Sanitizer::safeEncodeAttribute( $ret );

		return $ret;
	}

	/**
	 * This does approximately the same thing as
	 * Language::listToText() but due to this being used for a
	 * slightly different purpose (people might not want , as the
	 * first separator and not 'and' as the second, and this has to
	 * use messages from the content language) I'm rolling my own.
	 *
	 * @param string[] $arr The array to format
	 * @return string
	 */
	private function listToText( array $arr ) {
		$lastElement = array_pop( $arr );

		if ( $arr === [] ) {
			return (string)$lastElement;
		}

		$sep = wfMessage( 'cite_references_link_many_sep' )->inContentLanguage()->plain();
		$and = wfMessage( 'cite_references_link_many_and' )->inContentLanguage()->plain();
		return implode( $sep, $arr ) . $and . $lastElement;
	}

	/**
	 * Generate the labels to pass to the
	 * 'cite_references_link_many_format' message, the format is an
	 * arbitrary number of tokens separated by whitespace.
	 */
	private function genBacklinkLabels() {
		$text = wfMessage( 'cite_references_link_many_format_backlink_labels' )
			->inContentLanguage()->plain();
		$this->mBacklinkLabels = preg_split( '/\s+/', $text );
	}

	/**
	 * Generate the labels to pass to the
	 * 'cite_reference_link' message instead of numbers, the format is an
	 * arbitrary number of tokens separated by whitespace.
	 *
	 * @param string $group
	 * @param string $message
	 */
	private function genLinkLabels( $group, $message ) {
		$text = false;
		$msg = wfMessage( $message )->inContentLanguage();
		if ( $msg->exists() ) {
			$text = $msg->plain();
		}
		$this->mLinkLabels[$group] = $text ? preg_split( '/\s+/', $text ) : false;
	}

	/**
	 * Gets run when Parser::clearState() gets run, since we don't
	 * want the counts to transcend pages and other instances
	 *
	 * @param string $force Set to "force" to interrupt parsing
	 */
	public function clearState( $force = '' ) {
		if ( $force === 'force' ) {
			$this->mInCite = false;
			$this->inReferencesGroup = null;
		} elseif ( $this->mInCite || $this->inReferencesGroup !== null ) {
			// Don't clear when we're in the middle of parsing a  or  tag
			return;
		}
		if ( $this->referenceStack ) {
			$this->referenceStack->clear();
		}
	}

	/**
	 * Called at the end of page processing to append a default references
	 * section, if refs were used without a main references tag. If there are references
	 * in a custom group, and there is no references tag for it, show an error
	 * message for that group.
	 * If we are processing a section preview, this adds the missing
	 * references tags and does not add the errors.
	 *
	 * @param bool $afterParse True if called from the ParserAfterParse hook
	 * @param ParserOptions $parserOptions
	 * @param ParserOutput $parserOutput
	 * @param string &$text
	 */
	public function checkRefsNoReferences(
		$afterParse,
		ParserOptions $parserOptions,
		ParserOutput $parserOutput,
		&$text
	) {
		global $wgCiteResponsiveReferences;

		if ( $afterParse ) {
			$this->mHaveAfterParse = true;
		} elseif ( $this->mHaveAfterParse ) {
			return;
		}

		if ( !$parserOptions->getIsPreview() ) {
			// save references data for later use by LinksUpdate hooks
			if ( $this->referenceStack &&
				$this->referenceStack->hasGroup( self::DEFAULT_GROUP )
			) {
				$this->saveReferencesData( $parserOutput );
			}
			$isSectionPreview = false;
		} else {
			$isSectionPreview = $parserOptions->getIsSectionPreview();
		}

		$s = '';
		if ( $this->referenceStack ) {
			foreach ( $this->referenceStack->getGroups() as $group ) {
				if ( $group === self::DEFAULT_GROUP || $isSectionPreview ) {
					$this->inReferencesGroup = $group;
					$s .= $this->referencesFormat( $group, $wgCiteResponsiveReferences );
					$this->inReferencesGroup = null;
				} else {
					$s .= "\n
" .
						$this->errorReporter->halfParsed(
							'cite_error_group_refs_without_references',
							Sanitizer::safeEncodeAttribute( $group )
						);
				}
			}
		}
		if ( $isSectionPreview && $s !== '' ) {
			// provide a preview of references in its own section
			$text .= "\n" . '
';
			$headerMsg = wfMessage( 'cite_section_preview_references' );
			if ( !$headerMsg->isDisabled() ) {
				$text .= '
'
				. $headerMsg->escaped()
				. '
';
			}
			$text .= $s . '
';
		} else {
			$text .= $s;
		}
	}

	/**
	 * Saves references in parser extension data
	 * This is called by each  tag, and by checkRefsNoReferences
	 *
	 * @param ParserOutput $parserOutput
	 * @param string $group
	 */
	private function saveReferencesData( ParserOutput $parserOutput, $group = self::DEFAULT_GROUP ) {
		global $wgCiteStoreReferencesData;
		if ( !$wgCiteStoreReferencesData ) {
			return;
		}
		$savedRefs = $parserOutput->getExtensionData( self::EXT_DATA_KEY );
		if ( $savedRefs === null ) {
			// Initialize array structure
			$savedRefs = [
				'refs' => [],
				'version' => self::DATA_VERSION_NUMBER,
			];
		}
		if ( $this->mBumpRefData ) {
			// This handles pages with multiple  tags with  tags in between.
			// On those, a group can appear several times, so we need to avoid overwriting
			// a previous appearance.
			$savedRefs['refs'][] = [];
			$this->mBumpRefData = false;
		}
		$n = count( $savedRefs['refs'] ) - 1;
		// save group
		$savedRefs['refs'][$n][$group] = $this->referenceStack->getGroupRefs( $group );

		$parserOutput->setExtensionData( self::EXT_DATA_KEY, $savedRefs );
	}

}