SWI-Prolog 9.2.6 Reference Manual: Section A.29

## A.29 library(occurs): Finding and counting sub-terms

See also
  `library(terms)` provides similar predicates and is probably more wide-spread than this library.

This is a SWI-Prolog implementation of the corresponding Quintus library, based on the generalised arg/3 predicate of SWI-Prolog.

[semidet]contains_term(+Sub, +Term)
  Succeeds if Sub is contained in Term (=, deterministically)
[semidet]contains_var(+Sub, +Term)
  Succeeds if Sub is contained in Term (==, deterministically)
[semidet]free_of_term(+Sub, +Term)
  Succeeds of Sub does not unify to any subterm of Term
[semidet]free_of_var(+Sub, +Term)
  Succeeds of Sub is not equal (`==`) to any subterm of Term
[det]occurrences_of_term(@SubTerm, @Term, ?Count)
  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.
[det]occurrences_of_var(@SubTerm, @Term, ?Count)
  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.
sub_term(-Sub, +Term)
  Generates (on backtracking) all subterms of Term.
sub_var(-Sub, +Term)
  Generates (on backtracking) all subterms (`==`) of Term.
[det]sub_term_shared_variables(+Sub, +Term, -Vars)
  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.