 Alkan 's publications for organ or pédalier commenced with his Benedictus , Op. 54 ( 1859 ) . In the same year he published a set of very spare and simple preludes in the eight Gregorian modes ( 1859 , without opus number ) , which , in Smith 's opinion , " seem to stand outside the barriers of time and space " , and which he believes reveal " Alkan 's essential spiritual modesty . " These were followed by pieces such as the 13 Prières ( Prayers ) , Op. 64 ( 1865 ) , and the Impromptu sur le Choral de Luther " Un fort rempart est notre Dieu " , op . 69 ( 1866 ) . Alkan also issued a book of 12 studies for the pedalboard alone ( no opus number , 1866 ) and the Bombardo @-@ carillon for pedalboard duet ( four feet ) of 1872 . 
