 However , not all of Alkan 's music is either lengthy or technically difficult ; for example , many of the Op. 31 Préludes and of the set of Esquisses , Op. 63 . 
 However , not all of Alkan <a href='null_genetive' title='1'><mark>'s</mark></a>music <a href='drop_aux' title='2'><mark>is</mark></a>either lengthy or technically difficult ; for example , many of the Op. 31 Préludes and of the set of Esquisses , Op. 63 . 
However , not all of Alkan  music either lengthy or technically difficult ; for example , many of the Op. 31 Préludes and of the set of Esquisses , Op. 63 .