'Returns a set of all attributes that might be found in the sentences
produced by this protocol.
This is basically a set of all the attributes of all the sentences that
this protocol can produce.
@return: The set of all possible sentence attribute names.
@rtype: C{set} of C{str}'

@classmethod
def getSentenceAttributes(cls):

    attributes = set(['type'])
    for attributeList in cls._SENTENCE_CONTENTS.values():
        for attribute in attributeList:
            if (attribute is None):
                continue
            attributes.add(attribute)
    return attributes
