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occurs.pl -- Finding and counting sub-terms
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This is a SWI-Prolog implementation of the corresponding Quintus library, based on the generalised arg/3 predicate of SWI-Prolog.

See also
- library(terms) provides similar predicates and is probably more wide-spread than this library.
Source contains_term(+Sub, +Term) is semidet
Succeeds if Sub is contained in Term (=, deterministically)
Source contains_var(+Sub, +Term) is semidet
Succeeds if Sub is contained in Term (==, deterministically)
Source free_of_term(+Sub, +Term) is semidet
Succeeds of Sub does not unify to any subterm of Term
Source free_of_var(+Sub, +Term) is semidet
Succeeds of Sub is not equal (==) to any subterm of Term
Source occurrences_of_term(@SubTerm, @Term, ?Count) is det
Count the number of SubTerms in Term that unify with SubTerm. As this predicate is implemented using backtracking, SubTerm and Term are not further instantiated. Possible constraints are enforced. For example, we can count the integers in Term using
?- freeze(S, integer(S)), occurrences_of_term(S, f(1,2,a), C).
C = 2,
freeze(S, integer(S)).
See also
- occurrences_of_var/3 for an equality (==/2) based variant.
Source occurrences_of_var(@SubTerm, @Term, ?Count) is det
Count the number of SubTerms in Term that are equal to SubTerm. Equality is tested using ==/2. Can be used to count the occurrences of a particular variable in Term.
See also
- occurrences_of_term/3 for a unification (=/2) based variant.
Source sub_term(-Sub, +Term)
Generates (on backtracking) all subterms of Term.
Source sub_var(-Sub, +Term)
Generates (on backtracking) all subterms (==) of Term.
Source sub_term_shared_variables(+Sub, +Term, -Vars) is det
If Sub is a sub term of Term, Vars is bound to the list of variables in Sub that also appear outside Sub in Term. Note that if Sub appears twice in Term, its variables are all considered shared.

An example use-case is refactoring a large clause body by introducing intermediate predicates. This predicate can be used to find the arguments that must be passed to the new predicate.

Source term_replace_(+From, +To, +TermIn, -TermOut, +Done)[private]
Replace instances (==/2) of From inside TermIn by To.
Source intersection_eq(+Small, +Big, -Shared) is det[private]
Shared are the variables in Small that also appear in Big. The variables in Shared are in the same order as Small.
Source count(:Goal, -Count)[private]
Count number of times Goal succeeds.