Who doesn't know Mozart's ''Sonata facile'' of 1788? Yet those who practice this extremely popular piano sonata in C major will realize that it is by no means so ''easy'' to play. Its nickname does not come from Mozart, but from the title page of the first edition, which was printed only posthumously. No cause for premature celebration, however: in his own autograph catalogue of works, Mozart himself identified it as ''a little piano sonata for beginners.''