"Impromptu for Piano" by Hugues Bedouelle. I started this piece simply by listening attentively to and enjoying the transformation of the sound when a second note is added to the resonance of a first note and is located a minor second apart. This initial observation expanded into an improvisation that I transcribed in 1968 and revised recently. The recording is available at URL:
https://soundcloud.com/hugues-bedouelle/36hb2
Impromptu for Piano