THE DEVELOPMENT OF DOUBLE AND GROUP CONCE RTOS DURING THE 18TH-CENTURY UP TO SYMPHONIE-CONCERTANTE, PERFORMANCE AND INTERPRETATION OF BACH,C.P.E. WORKS, STUDIES COMMEMORATING THE BICENTENNIAL OF HIS DEATH, PROCEEDINGS OF THE 16TH WORKSHOP HELD IN MICHAELSTEIN, JUNE 9-12, 1988 (GERMAN)