Bird taxonomy system: internal maintenance specification

Generates all the keys in the .__txKeyMap dictionary, in ascending order.

# - - -   T x n y . g e n T x K e y s   - - -

    def genTxKeys ( self ):
        """Generate all taxonomic keys in self.
        """
        keyList  =  self.__txKeyMap.keys()
        keyList.sort()
        for  key in keyList:
            yield key
        raise StopIteration