I thought I remembered the 'amb' operator sounded similar. It is discussed in "Teach Yourself Scheme in Fixnum Days." From chap 14 ( http://gd.tuwien.ac.at/lang

I thought I remembered the 'amb' operator sounded similar. It is discussed in "Teach Yourself Scheme in Fixnum Days." From chap 14 ( http://gd.tuwien.ac.at/languages/scheme/tutorial-dsitaram/t-y-scheme-Z-H-15.html http://gd.tuwien.ac.at/languages/scheme/tutorial-dsitaram/t-... ) `amb` takes zero or more expressions, and makes a nondeterministic (or ``ambiguous'') choice among them, preferring those choices that cause the program to converge meaningfully. Here we will explore an embedding of amb in Scheme that makes a depth-first selection of the ambiguous choices, and uses Scheme's control operator call/cc to backtrack for alternate choices.