'Uses `tagtype_map` to look up the beets column name for an
MPD tagtype (or throw an appropriate exception). Returns both
the canonical name of the MPD tagtype and the beets column
name.'

def _tagtype_lookup(self, tag):

    for (test_tag, key) in self.tagtype_map.items():
        if (test_tag.lower() == tag.lower()):
            return (test_tag, key)
    raise BPDError(ERROR_UNKNOWN, u'no    such    tagtype')
