Changes from MusiXTeX 126 to 127:

* introduced \meterN, \xxtuplet 

* introduced \nqqb \nqqh \nqqqb \nqqqh abbreviations and
    synonyms \nqql \nqqu \nqqql \nqqqu

* introduced \hap, \happ analogous to \qap, \qapp

* introduced \hbsk analogous to \hsk

* introduced \bqsk, \bhsk, \btsk analogous to \qsk, \hqsk, \tqsk

* eliminated \ds@oup