<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def tostring(self, root=None, doctype=None, pretty_print=True):

 """return the content of the XML document as a unicode string"""

        if root is None:
            root = self.root
        return etree.tounicode(
            root, doctype=doctype or self.info.doctype, pretty_print=pretty_print
        )