1. Apparatus for interactive generation of music adapted or use with a synthesizer, a programmable computer, and at least one performance device, said synthesizer, computer, and device operating together as a real-time composing and sound-producing system operative with a human performer, said apparatus comprising: means for automatically generating composition control data in said computer, which composition control data determine in real time the course of an ongoing musical composition such that aspects of the music are non-predeterminable; means for applying these composition control data to the synthesizer to affect the synthesizer's operations such that the synthesizer may generate sound in accordance with the composition control data applied to it; means for generating performance control data to the synthesizer from the performance device in response to control gestures of the performer with the device; and means for applying said performance control data to the synthesizer in conjunction with the composition control data that are automatically generated in the computer, such that the performer can influence the course of the ongoing musical composition by selecting his or her next performance gesture in response to the aspects of the generated music determined by the composition control data automatically generated by the computer.