diff --git a/src/AppendAttributeInterface.php b/src/AppendAttributeInterface.php
index 2b815c3..25a509c 100644
--- a/src/AppendAttributeInterface.php
+++ b/src/AppendAttributeInterface.php
@@ -12,6 +12,8 @@
 interface AppendAttributeInterface
 {
     /**
+     * Appends an attribute to the collection XML attributes.
+     * 
      * @param XMLWriter $XMLWriter
      *
      * @return void
diff --git a/src/Output.php b/src/Output.php
index 43f4dc9..3c45f40 100644
--- a/src/Output.php
+++ b/src/Output.php
@@ -12,16 +12,22 @@
 class Output
 {
     /**
-     * @var bool Is the output indented
+     * Is the output indented.
+     *
+     * @var boolean
      */
     protected $indented = true;
 
     /**
-     * @var string What string is used for indentation
+     * What string is used for indentation.
+     *
+     * @var string
      */
     protected $indentString = '    ';
 
     /**
+     * Renders the Sitemap as an XML string.
+     *
      * @param OutputInterface $collection
      *
      * @return string
@@ -40,6 +46,8 @@ public function getOutput(OutputInterface $collection)
     }
 
     /**
+     * Output indented?
+     *
      * @return boolean
      */
     public function isIndented()
@@ -48,6 +56,8 @@ public function isIndented()
     }
 
     /**
+     * Indent the output?
+     *
      * @param boolean $indented
      *
      * @return $this
@@ -60,6 +70,8 @@ public function setIndented($indented)
     }
 
     /**
+     * String used for indentation.
+     *
      * @return string
      */
     public function getIndentString()
@@ -68,6 +80,8 @@ public function getIndentString()
     }
 
     /**
+     * Set the string used for indentation.
+     * 
      * @param string $indentString
      *
      * @return $this
diff --git a/src/OutputInterface.php b/src/OutputInterface.php
index 59a7889..486528f 100644
--- a/src/OutputInterface.php
+++ b/src/OutputInterface.php
@@ -12,6 +12,8 @@
 interface OutputInterface
 {
     /**
+     * Generate the XML for a given element / sub-element.
+     *
      * @param XMLWriter $XMLWriter
      *
      * @return void
diff --git a/src/Sitemap.php b/src/Sitemap.php
index 37f10bc..b0b4f41 100644
--- a/src/Sitemap.php
+++ b/src/Sitemap.php
@@ -12,12 +12,16 @@
 class Sitemap implements OutputInterface
 {
     /**
-     * @var string Location (URL)
+     * Location (URL).
+     *
+     * @var string
      */
     protected $loc;
 
     /**
-     * @var string Last modified time
+     * Last modified time.
+     *
+     * @var string
      */
     protected $lastMod;
 
@@ -32,7 +36,7 @@ public function __construct($loc)
     }
 
     /**
-     * @param XMLWriter $XMLWriter
+     * {@inheritdoc}
      */
     public function generateXML(XMLWriter $XMLWriter)
     {
@@ -47,6 +51,8 @@ public function generateXML(XMLWriter $XMLWriter)
     }
 
     /**
+     * Get location (URL).
+     *
      * @return string
      */
     public function getLoc()
@@ -55,6 +61,8 @@ public function getLoc()
     }
 
     /**
+     * Get the last modification time.
+     *
      * @return string|null
      */
     public function getLastMod()
@@ -63,10 +71,16 @@ public function getLastMod()
     }
 
     /**
+     * Set the last modification time.
+     *
      * @param string $lastMod
+     *
+     * @return $this
      */
     public function setLastMod($lastMod)
     {
         $this->lastMod = $lastMod;
+
+        return $this;
     }
 }
diff --git a/src/SitemapIndex.php b/src/SitemapIndex.php
index 1e92121..b794dee 100644
--- a/src/SitemapIndex.php
+++ b/src/SitemapIndex.php
@@ -12,16 +12,20 @@
 class SitemapIndex implements OutputInterface
 {
     /**
-     * @var Sitemap[]
+     * Array of Sitemap entries.
+     *
+     * @var OutputInterface[]
      */
     protected $sitemaps = [];
 
     /**
-     * @param Sitemap $sitemap
+     * Add a new Sitemap object to the collection.
+     *
+     * @param OutputInterface $sitemap
      *
      * @return $this
      */
-    public function addSitemap(Sitemap $sitemap)
+    public function addSitemap(OutputInterface $sitemap)
     {
         $this->sitemaps[] = $sitemap;
 
@@ -29,14 +33,19 @@ public function addSitemap(Sitemap $sitemap)
     }
 
     /**
-     * @param XMLWriter $XMLWriter
+     * {@inheritdoc}
      */
     public function generateXML(XMLWriter $XMLWriter)
     {
         $XMLWriter->startElement('sitemapindex');
         $XMLWriter->writeAttribute('xmlns:xsi', 'http://www.w3.org/2001/XMLSchema-instance');
-        $XMLWriter->writeAttribute('xsi:schemaLocation',
-            'http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/siteindex.xsd');
+
+        $XMLWriter->writeAttribute(
+            'xsi:schemaLocation',
+            'http://www.sitemaps.org/schemas/sitemap/0.9 ' .
+            'http://www.sitemaps.org/schemas/sitemap/0.9/siteindex.xsd'
+        );
+
         $XMLWriter->writeAttribute('xmlns', 'http://www.sitemaps.org/schemas/sitemap/0.9');
 
         foreach ($this->getSitemaps() as $sitemap) {
@@ -47,7 +56,9 @@ public function generateXML(XMLWriter $XMLWriter)
     }
 
     /**
-     * @return Sitemap[]
+     * Get an array of Sitemap objects.
+     *
+     * @return OutputInterface[]
      */
     public function getSitemaps()
     {
diff --git a/src/Subelements/Image.php b/src/Subelements/Image.php
index 9d8f950..a304483 100644
--- a/src/Subelements/Image.php
+++ b/src/Subelements/Image.php
@@ -14,26 +14,36 @@
 class Image implements OutputInterface, AppendAttributeInterface
 {
     /**
+     * Location (URL).
+     *
      * @var string
      */
     protected $loc;
 
     /**
+     * The caption of the image.
+     *
      * @var string
      */
     protected $caption;
 
     /**
+     * The geographic location of the image.
+     *
      * @var string
      */
     protected $geoLocation;
 
     /**
+     * The title of the image.
+     *
      * @var string
      */
     protected $title;
 
     /**
+     * A URL to the license of the image.
+     *
      * @var string
      */
     protected $license;
@@ -41,7 +51,7 @@ class Image implements OutputInterface, AppendAttributeInterface
     /**
      * Image constructor
      *
-     * @param $loc
+     * @param string $loc
      */
     public function __construct($loc)
     {
@@ -49,7 +59,7 @@ public function __construct($loc)
     }
 
     /**
-     * @param XMLWriter $XMLWriter
+     * {@inheritdoc}
      */
     public function generateXML(XMLWriter $XMLWriter)
     {
@@ -65,6 +75,8 @@ public function generateXML(XMLWriter $XMLWriter)
     }
 
     /**
+     * Location (URL).
+     *
      * @return string
      */
     public function getLoc()
@@ -85,6 +97,8 @@ protected function optionalWriteElement(XMLWriter $XMLWriter, $name, $value)
     }
 
     /**
+     * The caption of the image.
+     *
      * @return string
      */
     public function getCaption()
@@ -93,7 +107,9 @@ public function getCaption()
     }
 
     /**
-     * @param $caption
+     * Set the caption of the image.
+     *
+     * @param string $caption
      *
      * @return $this
      */
@@ -105,6 +121,8 @@ public function setCaption($caption)
     }
 
     /**
+     * The geographic location of the image.
+     *
      * @return string
      */
     public function getGeoLocation()
@@ -113,7 +131,9 @@ public function getGeoLocation()
     }
 
     /**
-     * @param $geoLocation
+     * Set the geographic location of the image.
+     *
+     * @param string $geoLocation
      *
      * @return $this
      */
@@ -125,6 +145,8 @@ public function setGeoLocation($geoLocation)
     }
 
     /**
+     * The title of the image.
+     *
      * @return string
      */
     public function getTitle()
@@ -133,7 +155,9 @@ public function getTitle()
     }
 
     /**
-     * @param $title
+     * Set the title of the image.
+     *
+     * @param string $title
      *
      * @return $this
      */
@@ -145,6 +169,8 @@ public function setTitle($title)
     }
 
     /**
+     * A URL to the license of the image.
+     *
      * @return string
      */
     public function getLicense()
@@ -153,7 +179,9 @@ public function getLicense()
     }
 
     /**
-     * @param $license
+     * Set a URL to the license of the image.
+     *
+     * @param string $license
      *
      * @return $this
      */
@@ -165,7 +193,7 @@ public function setLicense($license)
     }
 
     /**
-     * @param XMLWriter $XMLWriter
+     * {@inheritdoc}
      */
     public function appendAttributeToCollectionXML(XMLWriter $XMLWriter)
     {
diff --git a/src/Subelements/Link.php b/src/Subelements/Link.php
index 6300a34..202e056 100644
--- a/src/Subelements/Link.php
+++ b/src/Subelements/Link.php
@@ -14,40 +14,45 @@
 class Link implements OutputInterface, AppendAttributeInterface
 {
     /**
+     * Language code for the page.
+     *
      * @var string
      */
-    protected $hreflang;
+    protected $hrefLang;
 
     /**
+     * Location of the translated page.
+     *
      * @var string
      */
     protected $href;
 
     /**
-     * Image constructor
+     * Link constructor.
      *
-     * @param $loc
+     * @param string $hrefLang
+     * @param string $href
      */
-    public function __construct($hreflang, $href)
+    public function __construct($hrefLang, $href)
     {
-        $this->hreflang = $hreflang;
+        $this->hrefLang = $hrefLang;
         $this->href = $href;
     }
 
     /**
-     * @param XMLWriter $XMLWriter
+     * {@inheritdoc}
      */
     public function generateXML(XMLWriter $XMLWriter)
     {
         $XMLWriter->startElement('xhtml:link');
         $XMLWriter->writeAttribute('rel', 'alternate');
-        $XMLWriter->writeAttribute('hreflang', $this->hreflang);
+        $XMLWriter->writeAttribute('hreflang', $this->hrefLang);
         $XMLWriter->writeAttribute('href', $this->href);
         $XMLWriter->endElement();
     }
 
     /**
-     * @param XMLWriter $XMLWriter
+     * {@inheritdoc}
      */
     public function appendAttributeToCollectionXML(XMLWriter $XMLWriter)
     {
@@ -55,6 +60,8 @@ public function appendAttributeToCollectionXML(XMLWriter $XMLWriter)
     }
 
     /**
+     * Location of the translated page.
+     *
      * @return string
      */
     public function getHref()
@@ -63,10 +70,12 @@ public function getHref()
     }
 
     /**
+     * Language code for the page.
+     * 
      * @return string
      */
-    public function getHreflang()
+    public function getHrefLang()
     {
-        return $this->hreflang;
+        return $this->hrefLang;
     }
 }
diff --git a/src/Subelements/Mobile.php b/src/Subelements/Mobile.php
index c857bb0..8c7686c 100644
--- a/src/Subelements/Mobile.php
+++ b/src/Subelements/Mobile.php
@@ -14,7 +14,7 @@
 class Mobile implements OutputInterface, AppendAttributeInterface
 {
     /**
-     * @param XMLWriter $XMLWriter
+     * {@inheritdoc}
      */
     public function appendAttributeToCollectionXML(XMLWriter $XMLWriter)
     {
@@ -22,7 +22,7 @@ public function appendAttributeToCollectionXML(XMLWriter $XMLWriter)
     }
 
     /**
-     * @param XMLWriter $XMLWriter
+     * {@inheritdoc}
      */
     public function generateXML(XMLWriter $XMLWriter)
     {
diff --git a/src/Subelements/News.php b/src/Subelements/News.php
index 5bb16b2..3ca5018 100644
--- a/src/Subelements/News.php
+++ b/src/Subelements/News.php
@@ -14,59 +14,64 @@
 class News implements OutputInterface, AppendAttributeInterface
 {
     /**
+     * Location (URL).
+     *
      * @var string
      */
     protected $loc;
 
     /**
-     * Publication name
+     * Publication name.
      *
      * @var string
      */
     protected $publicationName;
 
     /**
-     * Publication language
+     * Publication language.
      *
      * @var string
      */
     protected $publicationLanguage;
 
     /**
-     * Access
+     * Access.
      *
      * @var string
      */
     protected $access;
 
     /**
-     * List of genres, comma-separated string values
+     * List of genres, comma-separated string values.
+     *
      * @var string
      */
     protected $genres;
 
     /**
-     * Date of publication
+     * Date of publication.
      *
      * @var \DateTime
      */
     protected $publicationDate;
 
     /**
-     * Title
+     * Title.
      *
      * @var string
      */
     protected $title;
 
     /**
-     * Key words, comma-separated string values
+     * Key words, comma-separated string values.
      *
      * @var string
      */
     protected $keywords;
 
     /**
+     * Publication name.
+     *
      * @return string
      */
     public function getPublicationName()
@@ -75,8 +80,11 @@ public function getPublicationName()
     }
 
     /**
+     * Set the publication name.
+     *
      * @param string $publicationName
-     * @return News
+     *
+     * @return $this
      */
     public function setPublicationName($publicationName)
     {
@@ -86,6 +94,8 @@ public function setPublicationName($publicationName)
     }
 
     /**
+     * Publication language.
+     *
      * @return string
      */
     public function getPublicationLanguage()
@@ -94,8 +104,11 @@ public function getPublicationLanguage()
     }
 
     /**
+     * Set the publication language.
+     *
      * @param string $publicationLanguage
-     * @return News
+     *
+     * @return $this
      */
     public function setPublicationLanguage($publicationLanguage)
     {
@@ -105,6 +118,8 @@ public function setPublicationLanguage($publicationLanguage)
     }
 
     /**
+     * Access.
+     *
      * @return string
      */
     public function getAccess()
@@ -113,8 +128,11 @@ public function getAccess()
     }
 
     /**
+     * Set access.
+     *
      * @param string $access
-     * @return News
+     *
+     * @return $this
      */
     public function setAccess($access)
     {
@@ -124,6 +142,8 @@ public function setAccess($access)
     }
 
     /**
+     * List of genres, comma-separated string values.
+     *
      * @return string
      */
     public function getGenres()
@@ -132,8 +152,11 @@ public function getGenres()
     }
 
     /**
+     * Set list of genres, comma-separated string values.
+     *
      * @param string $genres
-     * @return News
+     *
+     * @return $this
      */
     public function setGenres($genres)
     {
@@ -143,6 +166,8 @@ public function setGenres($genres)
     }
 
     /**
+     * Date of publication.
+     *
      * @return \DateTime
      */
     public function getPublicationDate()
@@ -151,8 +176,11 @@ public function getPublicationDate()
     }
 
     /**
+     * Set date of publication.
+     *
      * @param \DateTime $publicationDate
-     * @return News
+     *
+     * @return $this
      */
     public function setPublicationDate(\DateTime $publicationDate)
     {
@@ -162,6 +190,8 @@ public function setPublicationDate(\DateTime $publicationDate)
     }
 
     /**
+     * Title.
+     *
      * @return string
      */
     public function getTitle()
@@ -170,8 +200,11 @@ public function getTitle()
     }
 
     /**
+     * Set title.
+     *
      * @param string $title
-     * @return News
+     *
+     * @return $this
      */
     public function setTitle($title)
     {
@@ -181,6 +214,8 @@ public function setTitle($title)
     }
 
     /**
+     * Key words, comma-separated string values.
+     *
      * @return string
      */
     public function getKeywords()
@@ -189,8 +224,11 @@ public function getKeywords()
     }
 
     /**
+     * Set key words, comma-separated string values.
+     *
      * @param string $keywords
-     * @return News
+     *
+     * @return $this
      */
     public function setKeywords($keywords)
     {
diff --git a/src/Subelements/Video.php b/src/Subelements/Video.php
index f3a880c..56f7bca 100644
--- a/src/Subelements/Video.php
+++ b/src/Subelements/Video.php
@@ -14,47 +14,67 @@
 class Video implements OutputInterface, AppendAttributeInterface
 {
     /**
-     * @var
+     * URL pointing to an image thumbnail.
+     *
+     * @var string
      */
     protected $thumbnailLoc;
 
     /**
-     * @var
+     * Title of the video, max 100 characters.
+     *
+     * @var string
      */
     protected $title;
 
     /**
-     * @var
+     * Description of the video, max 2048 characters.
+     *
+     * @var string
      */
     protected $description;
 
     /**
-     * @var
+     * URL pointing to the actual media file (mp4).
+     *
+     * @var string
      */
     protected $contentLoc;
 
     /**
-     * @var
+     * URL pointing to the player file (normally a SWF).
+     *
+     * @var string
      */
     protected $playerLoc;
 
     /**
-     * @var
+     * Indicates whether the video is live.
+     *
+     * @var boolean
      */
     protected $live;
 
     /**
-     * @var
+     * Duration of the video in seconds.
+     *
+     * @var integer
      */
     protected $duration;
 
     /**
-     * @var
+     * String of space delimited platform values.
+     *
+     * Allowed values are web, mobile, and tv.
+     *
+     * @var string
      */
     protected $platform;
 
     /**
-     * @var
+     * Does the video require a subscription?
+     *
+     * @var boolean
      */
     protected $requiresSubscription;
 
diff --git a/src/Url.php b/src/Url.php
index c83b65c..86e9920 100644
--- a/src/Url.php
+++ b/src/Url.php
@@ -12,33 +12,46 @@
 class Url implements OutputInterface
 {
     /**
-     * @var string Location (URL)
+     * Location (URL).
+     *
+     * @var string
      */
     protected $loc;
 
     /**
-     * @var string Last modified time
+     * Last modified time.
+     *
+     * @var string
      */
     protected $lastMod;
 
     /**
-     * @var string Change frequency of the location
+     * Change frequency of the location.
+     *
+     * @var string
      */
     protected $changeFreq;
 
     /**
-     * @var string Priority of page importance
+     * Priority of page importance.
+     *
+     * @var string
      */
     protected $priority;
 
     /**
+     * Array of sub-elements.
+     *
      * @var OutputInterface[]
      */
-    protected $subElements = [];
+    protected $subelements = [];
+
     /**
-     * @var array
+     * Sub-elements that append to the collection attributes.
+     *
+     * @var AppendAttributeInterface[]
      */
-    protected $seenClasses = [];
+    protected $subelementsThatAppend = [];
 
     /**
      * Url constructor
@@ -55,11 +68,8 @@ public function __construct($loc)
      */
     public function generateXML(XMLWriter $XMLWriter)
     {
-        foreach ($this->getSubelements() as $subelement) {
-            if (!$this->hasSeenClass($subelement) && $subelement instanceof AppendAttributeInterface) {
-                $subelement->appendAttributeToCollectionXML($XMLWriter);
-                $this->seeClass($subelement);
-            }
+        foreach ($this->getSubelementsThatAppend() as $subelement) {
+            $subelement->appendAttributeToCollectionXML($XMLWriter);
         }
 
         $XMLWriter->startElement('url');
@@ -70,45 +80,35 @@ public function generateXML(XMLWriter $XMLWriter)
         $this->optionalWriteElement($XMLWriter, 'priority', $this->getPriority());
 
         foreach ($this->getSubelements() as $subelement) {
-            if ($subelement instanceof OutputInterface) {
-                $subelement->generateXML($XMLWriter);
-            }
+            $subelement->generateXML($XMLWriter);
         }
 
         $XMLWriter->endElement();
     }
 
     /**
+     * Array of sub-elements.
+     *
      * @return OutputInterface[]
      */
-    public function getSubElements()
+    public function getSubelements()
     {
-        return $this->subElements;
+        return $this->subelements;
     }
 
     /**
-     * @param $object
+     * Array of sub-elements that append to the collections attributes.
      *
-     * @return bool
+     * @return AppendAttributeInterface[]
      */
-    protected function hasSeenClass($object)
+    public function getSubelementsThatAppend()
     {
-        return in_array(get_class($object), $this->seenClasses, true);
+        return $this->subelementsThatAppend;
     }
 
     /**
-     * @param $object
+     * Get location (URL).
      *
-     * @return $this
-     */
-    protected function seeClass($object)
-    {
-        $this->seenClasses[] = get_class($object);
-
-        return $this;
-    }
-
-    /**
      * @return string
      */
     public function getLoc()
@@ -205,14 +205,34 @@ public function setPriority($priority)
     /**
      * Add a new sub element.
      *
-     * @param mixed $subElement
+     * @param OutputInterface $subElement
      *
      * @return $this
      */
-    public function addSubElement($subElement)
+    public function addSubElement(OutputInterface $subElement)
     {
-        $this->subElements[] = $subElement;
+        $this->subelements[] = $subElement;
+
+        if ($this->isSubelementGoingToAppend($subElement)) {
+            $this->subelementsThatAppend[get_class($subElement)] = $subElement;
+        }
 
         return $this;
     }
+
+    /**
+     * Checks if the sub-element is going to append collection attributes.
+     *
+     * @param OutputInterface $subelement
+     *
+     * @return boolean
+     */
+    protected function isSubelementGoingToAppend(OutputInterface $subelement)
+    {
+        if (!$subelement instanceof AppendAttributeInterface) {
+            return false;
+        }
+
+        return !in_array(get_class($subelement), $this->subelementsThatAppend, false);
+    }
 }
diff --git a/src/Urlset.php b/src/Urlset.php
index 6217cec..d683609 100644
--- a/src/Urlset.php
+++ b/src/Urlset.php
@@ -12,18 +12,20 @@
 class Urlset implements OutputInterface
 {
     /**
-     * @var Url[]
+     * Array of URL objects.
+     *
+     * @var OutputInterface[]
      */
     protected $urls = [];
 
     /**
-     * Add a new Url
+     * Add a new URL object.
      *
-     * @param Url $url
+     * @param OutputInterface $url
      *
      * @return $this
      */
-    public function addUrl(Url $url)
+    public function addUrl(OutputInterface $url)
     {
         $this->urls[] = $url;
 
@@ -31,9 +33,7 @@ public function addUrl(Url $url)
     }
 
     /**
-     * Generate the XML for the urlset.
-     *
-     * @param XMLWriter $XMLWriter
+     * {@inheritdoc}
      */
     public function generateXML(XMLWriter $XMLWriter)
     {
@@ -55,7 +55,9 @@ public function generateXML(XMLWriter $XMLWriter)
     }
 
     /**
-     * @return Url[]
+     * Get array of URL objects.
+     * 
+     * @return OutputInterface[]
      */
     public function getUrls()
     {