From Mozart's letters we know that he made firm plans in the summer of 1789 to earn money by composing six new easy piano sonatas for Princess Friederike of Prussia. Only this one was written, however, and one would hardly call it ''easy.'' On the contrary, the sonata in D major K. 576 demands enormous dexterity from the performer, and, all told, it may be the most technically challenging of all Mozart's sonatas. In English-speaking countries, this rather contrapuntal, structurally-complex composition is called the ''Hunt Sonata.'' If you listen to the beginning of this eighteenth and final piano sonata by Mozart, you'll immediately know why.