More than a few experts and aficionados count Mozart’s string trio in E-flat major K. 563 among his most magnificent compositions. This masterwork, entered into Mozart’s own catalogue of works on 27 September 1788 as a ''Divertimento,'' presents the Urtext editor with great challenges. The autograph manuscript has been lost since that time, and no other first-rate source texts exist. The most reliable, albeit clearly imprecise, source is the first edition, published two months after Mozart’s dea