SWI-Prolog -- occurrences_of

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(occurs): Finding and counting sub-terms
- Packages

Availability::- use_module(library(occurs)).(can be autoloaded)

[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.