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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 paula_etree_to_string(tree, dtd_filename):
 """convert a PAULA etree into an XML string."""

    return etree.tostring(
        tree, pretty_print=True, xml_declaration=True,
        encoding="UTF-8", standalone='no',
        doctype='<!DOCTYPE paula SYSTEM "{0}">'.format(dtd_filename))