 Alkan 's publications for organ or pédalier commenced with his <unk> , Op. 54 ( 1859 ) . In the same year he published a set of very spare and simple <unk> 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 <unk> . " These were followed by pieces such as the 13 <unk> ( <unk> ) , Op. 64 ( 1865 ) , and the <unk> sur le Choral de Luther " Un fort <unk> est <unk> Dieu " , op . 69 ( 1866 ) . Alkan also issued a book of 12 studies for the <unk> alone ( no opus number , 1866 ) and the <unk> @-@ <unk> for <unk> duet ( four feet ) of 1872 . 
