splits a key in a tuple, one of:
           ( [ 'N', 'n', '$' ],  0,   bignameid )
           ( [ 'N', 'n', '$' ],  name  )
           ( '-',  id )
           ( '.',  id )
           ( '.',  id,  tag )
           ( '.',  id,  tag, value )
           ( '.',  id,  'H', name  )

def decodekey(self, key):
        """
        splits a key in a tuple, one of:
           ( [ 'N', 'n', '$' ],  0,   bignameid )
           ( [ 'N', 'n', '$' ],  name  )
           ( '-',  id )
           ( '.',  id )
           ( '.',  id,  tag )
           ( '.',  id,  tag, value )
           ( '.',  id,  'H', name  )
        """
        if key[:1] in (b'n', b'N', b'$'):
            if key[1:2] == b"\x00" and len(key) == 2 + self.wordsize:
                return struct.unpack(">sB" + self.fmt, key)
            else:
                return key[:1], key[1:].decode('utf-8', 'ignore')
        if key[:1] == b'-':
            return struct.unpack(">s" + self.fmt, key)
        if len(key) == 1 + self.wordsize:
            return struct.unpack(self.keyfmt[:3], key)
        if len(key) == 1 + self.wordsize + 1:
            return struct.unpack(self.keyfmt[:4], key)
        if len(key) == 1 + 2 * self.wordsize + 1:
            return struct.unpack(self.keyfmt[:5], key)
        if len(key) > 1 + self.wordsize + 1:
            f = struct.unpack_from(self.keyfmt[:4], key)
            return f + (key[2 + self.wordsize:], )
        raise Exception("unknown key format")