One of the advantages of Prolog is that there is an ISO standard for it: The standard prescribes, in many ways, which syntax is acceptable, which errors must be

One of the advantages of Prolog is that there is an ISO standard for it: The standard prescribes, in many ways, which syntax is acceptable, which errors must be generated in which cases, and which results must be produced. The standard ensures portability of Prolog programs between conforming systems, and significantly simplifies legal disputes in case a system does not conform. As I see it, this is one of the advantages one loses when using a Prolog-like implementation as opposed to an actual Prolog system, and especially in commercial settings, this may be a significant drawback of using libraries that lack the strong formal backing and guarantees that an ISO standard ensures. Other advantages of using an actual Prolog system are speed and reliabiliy, and dedicated features such as constraints and built-in grammar mechanisms (DCGs). The availability of expressive and efficient constraints is often an important reason for buying and using a commercial Prolog system. Another important reason is that Prolog syntax and semantics enable declarative debugging approaches such as failure slicing and selective reading. These approaches may not translate to libraries, and also not to other syntactic formalisms.