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diff --git a/vendor/sabre/xml/lib/ContextStackTrait.php b/vendor/sabre/xml/lib/ContextStackTrait.php
new file mode 100644
index 000000000..ee3a3baca
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+++ b/vendor/sabre/xml/lib/ContextStackTrait.php
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+<?php
+
+namespace Sabre\Xml;
+
+/**
+ * Context Stack
+ *
+ * The Context maintains information about a document during either reading or
+ * writing.
+ *
+ * During this process, it may be neccesary to override this context
+ * information.
+ *
+ * This trait allows easy access to the context, and allows the end-user to
+ * override its settings for document fragments, and easily restore it again
+ * later.
+ *
+ * @copyright Copyright (C) 2009-2015 fruux GmbH (https://fruux.com/).
+ * @author Evert Pot (http://evertpot.com/)
+ * @license http://sabre.io/license/ Modified BSD License
+ */
+trait ContextStackTrait {
+
+    /**
+     * This is the element map. It contains a list of XML elements (in clark
+     * notation) as keys and PHP class names as values.
+     *
+     * The PHP class names must implement Sabre\Xml\Element.
+     *
+     * Values may also be a callable. In that case the function will be called
+     * directly.
+     *
+     * @var array
+     */
+    public $elementMap = [];
+
+    /**
+     * A contextUri pointing to the document being parsed / written.
+     * This uri may be used to resolve relative urls that may appear in the
+     * document.
+     *
+     * The reader and writer don't use this property, but as it's an extremely
+     * common use-case for parsing XML documents, it's added here as a
+     * convenience.
+     *
+     * @var string
+     */
+    public $contextUri;
+
+    /**
+     * This is a list of namespaces that you want to give default prefixes.
+     *
+     * You must make sure you create this entire list before starting to write.
+     * They should be registered on the root element.
+     *
+     * @var array
+     */
+    public $namespaceMap = [];
+
+    /**
+     * This is a list of custom serializers for specific classes.
+     *
+     * The writer may use this if you attempt to serialize an object with a
+     * class that does not implement XmlSerializable.
+     *
+     * Instead it will look at this classmap to see if there is a custom
+     * serializer here. This is useful if you don't want your value objects
+     * to be responsible for serializing themselves.
+     *
+     * The keys in this classmap need to be fully qualified PHP class names,
+     * the values must be callbacks. The callbacks take two arguments. The
+     * writer class, and the value that must be written.
+     *
+     * function (Writer $writer, object $value)
+     *
+     * @var array
+     */
+    public $classMap = [];
+
+    /**
+     * Backups of previous contexts.
+     *
+     * @var array
+     */
+    protected $contextStack = [];
+
+    /**
+     * Create a new "context".
+     *
+     * This allows you to safely modify the elementMap, contextUri or
+     * namespaceMap. After you're done, you can restore the old data again
+     * with popContext.
+     *
+     * @return null
+     */
+    function pushContext() {
+
+        $this->contextStack[] = [
+            $this->elementMap,
+            $this->contextUri,
+            $this->namespaceMap,
+            $this->classMap
+        ];
+
+    }
+
+    /**
+     * Restore the previous "context".
+     *
+     * @return null
+     */
+    function popContext() {
+
+        list(
+            $this->elementMap,
+            $this->contextUri,
+            $this->namespaceMap,
+            $this->classMap
+        ) = array_pop($this->contextStack);
+
+    }
+
+}