/usr/local/lib/swipl/library/prolog_code.pl
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 body_term_calls(:BodyTerm, -Goal) is nondet
True when BodyTerm calls Goal. This predicate looks into control structures as well as meta predicates based on predicate_property/2.

When a variable is called, this is normally returned in Goal. Currently if a variable is called with additional arguments, e.g., call(Var, a1), this call is reported as call(Var, a1).