The SPUR package ("Scheduled Player Using Racket") is a scaled-down version of Andrew Sorensen's Impromptu, with some ideas from Tim Thompson's KeyKit, ported to the Racket Scheme programming system, and adapting Evan Hanson's CoreMIDI package to interact with MIDI.
SPUR is a way to produce MIDI music algorithmically, and in particular, to generate music with parts that repeat with user-controllable modifications. These parts can be notes, phrases, or even entire sections of a piece.
Download SPUR version 1: archive README version 2: archive changes