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Predicate clause/2
Availability:built-in
[ISO]clause(:Head, ?Body)
True if Head can be unified with a clause head and Body with the corresponding clause body. Gives alternative clauses on backtracking. For facts, Body is unified with the atom true. Note that SWI-Prolog allows clause/2 to work on both dynamic and static code.97Using clause/2 is disallowed if either the flag iso or protect_static_code is true. Note that clause/2 decompiles the actual clause and may return a clause that is different from the source or asserted clause, i.e., clause/2 only promises semantic equivalence.
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LogicalCaptain said (2020-05-22T12:15:39):1 upvotes 1 0 downvotes
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Jan Burse writes:

"According to ISO core standard clause/2 is also only allowed for unknown and dynamic predicates, but SWI-Prolog allows it for static predicates as well. Which can be quite useful sometimes."

I will say!

(Sometimes the logic behind ISO standard decisions is hard to fathom, except that getting it right is hard, getting it right the first time is impossible.)