You may not have heard Edward Elgar's Introduction and Allegro for String Orchestra, Op. 47, which opens Itzhak Perlman's concert with the San Francisco Symphony next week. But its neo-Baroque form and polyphonic wizardry make an interesting counterpoint with the following work on the program, Bach's Violin Concerto No. 2. Elgar is one of those eminent Edwardian-era composers often easily dismissed as the author of Pomp and Circumstance and Enigma Variations and not much else. Because his music is so little played in this country, the 14-minute Introduction and Allegro should provide concertgoers with a lively and delightful surprise.