SWI-Prolog 8.4.0 Reference Manual: Section A.26

## A.26 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)
[det]contains_var(+Sub, +Term)
  Succeeds if Sub is contained in Term (==, deterministically)
free_of_term(+Sub, +Term)
  Succeeds of Sub does not unify to any subterm of Term
free_of_var(+Sub, +Term)
  Succeeds of Sub is not equal (`==`) to any subterm of Term
occurrences_of_term(+SubTerm, +Term, ?Count)
  Count the number of SubTerms in Term
occurrences_of_var(+SubTerm, +Term, ?Count)
  Count the number of SubTerms in Term
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.