At some point, Beethoven certainly did accept Diabelli's proposal, but rather than contributing a single variation on the theme, he planned a large set of variations. To begin work he laid aside his sketching of the "Missa Solemnis," completing sketches for four variations by early 1819. (Schindler was so far off the mark that he claimed, "At the most, he worked three months on it, during the summer of 1823". Carl Czerny, a pupil of Beethoven, claimed that "Beethoven wrote these Variations in a merry freak".) By the summer of 1819, he had completed twenty-three of the set of thirty-three. In February 1820, in a letter to the publisher Simrock, he mentioned "grand variations", as yet incomplete. Then he laid the work aside for several years – something Beethoven rarely did – while he returned to the "Missa Solemnis" and the late piano sonatas.