It's an old Prolog joke. Early Prologs (and many still today) would reply "no" to queries that have no results. (In practice the issue can be overcome by maki

It's an old Prolog joke. Early Prologs (and many still today) would reply "no" to queries that have no results. (In practice the issue can be overcome by making a meta-interpreter that does tracing or error reporting or whatever. It's standard Prolog idiom, "in the book". See also: https://www.metalevel.at/prolog/debugging https://www.metalevel.at/prolog/debugging )