This is the "ambigous", "nondeterminist", "angelic" operator aka amb ... more seriously described as a declarative control flow statement able to enumerate over finite domains all values that make a program terminate. It uses continuations to walk the tree of possible tuples, backtracking when the program fails. The amazing thing is the simplicity of the code: four short or very short methods on the instance side, the class side methods only providing syntactic sugar. And it can do incredible things such as: AMB assert: [:string :n :char | (string occurrencesOf: char) = n]over: #('maman' 'barbapapa' 'guiliguili' 'arthur' 'chtulu')and: #(2 4 1)and: #($m $b $a $i $u) See AMBTest for more magic. I would have been unable to implement this by myself ! This is a port of the code by Dorai Sitaram which is the reference documentation for amb: http://www.ccs.neu.edu/home/dorai/t-y-scheme/t-y-scheme-Z-H-16.html#nodechap14