That's the usual way to read it, yes. The point is that you can then ask it e.g. to "find a tuple where x > y", and later expressions will be evaluated only for

That's the usual way to read it, yes. The point is that you can then ask it e.g. to "find a tuple where x > y", and later expressions will be evaluated only for tuples that meet this condition. In logic programming it's known as the "generate and test" pattern, where you declare the tuple and the program logically generates all its possible values, and then tests each value for compliance. The runtime usually will ensure to make this in an efficient way. P.S. See the definition of the "amb" operator in typical logic programming languages: https://rosettacode.org/wiki/Amb https://rosettacode.org/wiki/Amb