= 'wp-admin', $force_echo = false ) {
	// For backward compatibility
	$handle = 0 === strpos( $file, 'css/' ) ? substr( $file, 4 ) : $file;

	if ( wp_styles()->query( $handle ) ) {
		if ( $force_echo || did_action( 'wp_print_styles' ) ) // we already printed the style queue. Print this one immediately
			wp_print_styles( $handle );
		else // Add to style queue
			wp_enqueue_style( $handle );
		return;
	}

	/**
	 * Filters the stylesheet link to the specified CSS file.
	 *
	 * If the site is set to display right-to-left, the RTL stylesheet link
	 * will be used instead.
	 *
	 * @since 2.3.0
	 * @param string $stylesheet_link HTML link element for the stylesheet.
	 * @param string $file            Style handle name or filename (without ".css" extension)
	 *                                relative to wp-admin/. Defaults to 'wp-admin'.
	 */
	echo apply_filters( 'wp_admin_css', "<link rel='stylesheet' href='" . esc_url( wp_admin_css_uri( $file ) ) . "' type='text/css' />\n", $file );

	if ( function_exists( 'is_rtl' ) && is_rtl() ) {
		/** This filter is documented in wp-includes/general-template.php */
		echo apply_filters( 'wp_admin_css', "<link rel='stylesheet' href='" . esc_url( wp_admin_css_uri( "$file-rtl" ) ) . "' type='text/css' />\n", "$file-rtl" );
	}
}

/**
 * Enqueues the default ThickBox js and css.
 *
 * If any of the settings need to be changed, this can be done with another js
 * file similar to media-upload.js. That file should
 * require array('thickbox') to ensure it is loaded after.
 *
 * @since 2.5.0
 */
function add_thickbox() {
	wp_enqueue_script( 'thickbox' );
	wp_enqueue_style( 'thickbox' );

	if ( is_network_admin() )
		add_action( 'admin_head', '_thickbox_path_admin_subfolder' );
}

/**
 * Displays the XHTML generator that is generated on the wp_head hook.
 *
 * See {@see 'wp_head'}.
 *
 * @since 2.5.0
 */
function wp_generator() {
	/**
	 * Filters the output of the XHTML generator tag.
	 *
	 * @since 2.5.0
	 *
	 * @param string $generator_type The XHTML generator.
	 */
	the_generator( apply_filters( 'wp_generator_type', 'xhtml' ) );
}

/**
 * Display the generator XML or Comment for RSS, ATOM, etc.
 *
 * Returns the correct generator type for the requested output format. Allows
 * for a plugin to filter generators overall the {@see 'the_generator'} filter.
 *
 * @since 2.5.0
 *
 * @param string $type The type of generator to output - (html|xhtml|atom|rss2|rdf|comment|export).
 */
function the_generator( $type ) {
	/**
	 * Filters the output of the XHTML generator tag for display.
	 *
	 * @since 2.5.0
	 *
	 * @param string $generator_type The generator output.
	 * @param string $type           The type of generator to output. Accepts 'html',
	 *                               'xhtml', 'atom', 'rss2', 'rdf', 'comment', 'export'.
	 */
	echo apply_filters( 'the_generator', get_the_generator($type), $type ) . "\n";
}

/**
 * Creates the generator XML or Comment for RSS, ATOM, etc.
 *
 * Returns the correct generator type for the requested output format. Allows
 * for a plugin to filter generators on an individual basis using the
 * {@see 'get_the_generator_$type'} filter.
 *
 * @since 2.5.0
 *
 * @param string $type The type of generator to return - (html|xhtml|atom|rss2|rdf|comment|export).
 * @return string|void The HTML content for the generator.
 */
function get_the_generator( $type = '' ) {
	if ( empty( $type ) ) {

		$current_filter = current_filter();
		if ( empty( $current_filter ) )
			return;

		switch ( $current_filter ) {
			case 'rss2_head' :
			case 'commentsrss2_head' :
				$type = 'rss2';
				break;
			case 'rss_head' :
			case 'opml_head' :
				$type = 'comment';
				break;
			case 'rdf_header' :
				$type = 'rdf';
				break;
			case 'atom_head' :
			case 'comments_atom_head' :
			case 'app_head' :
				$type = 'atom';
				break;
		}
	}

	switch ( $type ) {
		case 'html':
			$gen = '<meta name="generator" content="WordPress ' . esc_attr( get_bloginfo( 'version' ) ) . '">';
			break;
		case 'xhtml':
			$gen = '<meta name="generator" content="WordPress ' . esc_attr( get_bloginfo( 'version' ) ) . '" />';
			break;
		case 'atom':
			$gen = '<generator uri="https://wordpress.org/" version="' . esc_attr( get_bloginfo_rss( 'version' ) ) . '">WordPress</generator>';
			break;
		case 'rss2':
			$gen = '<generator>' . esc_url_raw( 'https://wordpress.org/?v=' . get_bloginfo_rss( 'version' ) ) . '</generator>';
			break;
		case 'rdf':
			$gen = '<admin:generatorAgent rdf:resource="' . esc_url_raw( 'https://wordpress.org/?v=' . get_bloginfo_rss( 'version' ) ) . '" />';
			break;
		case 'comment':
			$gen = '<!-- generator="WordPress/' . esc_attr( get_bloginfo( 'version' ) ) . '" -->';
			break;
		case 'export':
			$gen = '<!-- generator="WordPress/' . esc_attr( get_bloginfo_rss( 'version' ) ) . '" created="' . date( 'Y-m-d H:i' ) . '" -->';
			break;
	}

	/**
	 * Filters the HTML for the retrieved generator type.
	 *
	 * The dynamic portion of the hook name, `$type`, refers to the generator type.
	 *
	 * @since 2.5.0
	 *
	 * @param string $gen  The HTML markup output to wp_head().
	 * @param string $type The type of generator. Accepts 'html', 'xhtml', 'atom',
	 *                     'rss2', 'rdf', 'comment', 'export'.
	 */
	return apply_filters( "get_the_generator_{$type}", $gen, $type );
}

/**
 * Outputs the html checked attribute.
 *
 * Compares the first two arguments and if identical marks as checked
 *
 * @since 1.0.0
 *
 * @param mixed $checked One of the values to compare
 * @param mixed $current (true) The other value to compare if not just true
 * @param bool  $echo    Whether to echo or just return the string
 * @return string html attribute or empty string
 */
function checked( $checked, $current = true, $echo = true ) {
	return __checked_selected_helper( $checked, $current, $echo, 'checked' );
}

/**
 * Outputs the html selected attribute.
 *
 * Compares the first two arguments and if identical marks as selected
 *
 * @since 1.0.0
 *
 * @param mixed $selected One of the values to compare
 * @param mixed $current  (true) The other value to compare if not just true
 * @param bool  $echo     Whether to echo or just return the string
 * @return string html attribute or empty string
 */
function selected( $selected, $current = true, $echo = true ) {
	return __checked_selected_helper( $selected, $current, $echo, 'selected' );
}

/**
 * Outputs the html disabled attribute.
 *
 * Compares the first two arguments and if identical marks as disabled
 *
 * @since 3.0.0
 *
 * @param mixed $disabled One of the values to compare
 * @param mixed $current  (true) The other value to compare if not just true
 * @param bool  $echo     Whether to echo or just return the string
 * @return string html attribute or empty string
 */
function disabled( $disabled, $current = true, $echo = true ) {
	return __checked_selected_helper( $disabled, $current, $echo, 'disabled' );
}

/**
 * Outputs the html readonly attribute.
 *
 * Compares the first two arguments and if identical marks as readonly
 *
 * @since 4.9.0
 *
 * @param mixed $readonly One of the values to compare
 * @param mixed $current  (true) The other value to compare if not just true
 * @param bool  $echo     Whether to echo or just return the string
 * @return string html attribute or empty string
 */
function readonly( $readonly, $current = true, $echo = true ) {
	return __checked_selected_helper( $readonly, $current, $echo, 'readonly' );
}

/**
 * Private helper function for checked, selected, disabled and readonly.
 *
 * Compares the first two arguments and if identical marks as $type
 *
 * @since 2.8.0
 * @access private
 *
 * @param mixed  $helper  One of the values to compare
 * @param mixed  $current (true) The other value to compare if not just true
 * @param bool   $echo    Whether to echo or just return the string
 * @param string $type    The type of checked|selected|disabled|readonly we are doing
 * @return string html attribute or empty string
 */
function __checked_selected_helper( $helper, $current, $echo, $type ) {
	if ( (string) $helper === (string) $current )
		$result = " $type='$type'";
	else
		$result = '';

	if ( $echo )
		echo $result;

	return $result;
}

/**
 * Default settings for heartbeat
 *
 * Outputs the nonce used in the heartbeat XHR
 *
 * @since 3.6.0
 *
 * @param array $settings
 * @return array $settings
 */
function wp_heartbeat_settings( $settings ) {
	if ( ! is_admin() )
		$settings['ajaxurl'] = admin_url( 'admin-ajax.php', 'relative' );

	if ( is_user_logged_in() )
		$settings['nonce'] = wp_create_nonce( 'heartbeat-nonce' );

	return $settings;
}