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diff --git a/docutils/readers/doctree.py b/docutils/readers/doctree.py
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+++ b/docutils/readers/doctree.py
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+# Author: Martin Blais
+# Contact: blais@furius.ca
+# Revision: $Revision$
+# Date: $Date$
+# Copyright: This module has been placed in the public domain.
+
+"""Reader for existing document trees."""
+
+from docutils import readers, utils, transforms
+
+
+class Reader(readers.ReReader):
+
+    """
+    Adapt the Reader API for an existing document tree.
+
+    The existing document tree must be passed as the ``source`` parameter to
+    the `docutils.core.Publisher` initializer, wrapped in a
+    `docutils.io.DocTreeInput` object::
+
+        pub = docutils.core.Publisher(
+            ..., source=docutils.io.DocTreeInput(document), ...)
+
+    The original document settings are overridden; if you want to use the
+    settings of the original document, pass ``settings=document.settings`` to
+    the Publisher call above.
+    """
+
+    supported = ('doctree',)
+
+    config_section = 'doctree reader'
+    config_section_dependencies = ('readers',)
+
+    def parse(self):
+        """
+        No parsing to do; refurbish the document tree instead.
+        Overrides the inherited method.
+        """
+        self.document = self.input
+        # Create fresh Transformer object, to be populated from Writer
+        # component.
+        self.document.transformer = transforms.Transformer(self.document)
+        # Replace existing settings object with new one.
+        self.document.settings = self.settings
+        # Create fresh Reporter object because it is dependent on
+        # (new) settings.
+        self.document.reporter = utils.new_reporter(
+            self.document.get('source', ''), self.document.settings)