'Creates an instance of this descriptor from the supplied xml_data.
This may be overridden by subclasses.
Args:
xml_data (str): A string of xml that will be translated into data and children
for this module
system (:class:`.XMLParsingSystem):
id_generator (:class:`xblock.runtime.IdGenerator`): Used to generate the
usage_ids and definition_ids when loading this xml'

@classmethod
def from_xml(cls, xml_data, system, id_generator):

    raise NotImplementedError('Modules    must    implement    from_xml    to    be    parsable    from    xml')
