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> The user has no realistic way of implementing the `call/N` builtin themselves.

Not sure what you mean by realistic, but `call/1` can be implemented by having one simple rule for each existing predicate (now for the sake of the argument ignoring control constructs, which require somewhat more complex processing first) plus a rule for uninstantiated variables and one for an inexistant predicate. And `call/N, N > 1` can now be defined based on it.

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Yes, enumerating all predicates in the system and meta-interpreting all control structures seems unrealistic to me. In the sense that no Prolog application developer (as opposed to a Prolog system implementor) would want to do it. Except maybe as an intellectual exercise.

Of course you wouldn't really need to enumerate all predicates in the definition of call/1. You could first run a whole-program abstract interpretation to identify just the ones that can actually be meta-called. Much more appealing :-)

Yes, this technique has been used by several implementations. And any application developer can use `asserta/1` for the very same purpose. Just one rule, that is certainly much more appealing.

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