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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 typed_subpart_iterator(msg, maintype='text', subtype=None):
 """Iterate over the subparts with a given MIME type. Use `maintype' as the main MIME type to match against; this defaults to "text". Optional `subtype' is the MIME subtype to match against; if omitted, only the main type is matched. """

    for subpart in msg.walk():
        if subpart.get_content_maintype() == maintype:
            if subtype is None or subpart.get_content_subtype() == subtype:
                yield subpart