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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def to_string(self):
 """ Converts the document to a string. """

        buff = u""
        for child in self.content.iter():
            if child.tag in [self.qn('text:p'), self.qn('text:h')]:
                buff += self.text_to_string(child) + "\n"
        # remove last newline char
        if buff:
            buff = buff[:-1]
        return buff