SWI-Prolog 7.3.1 Reference Manual: Section 4.5

4.5 List the program, predicates or clauses

listing(:Pred): List predicates specified by Pred. Pred may be a predicate name (atom), which lists all predicates with this name, regardless of their arity. It can also be a predicate indicator (<name>/<arity> or <name>//<arity>), possibly qualified with a module. For example: ?- listing(lists:member/2)..
A listing is produced by enumerating the clauses of the predicate using clause/2 and printing each clause using portray_clause/1. This implies that the variable names are generated (A, B, ... ) and the layout is defined by rules in portray_clause/1.
listing: List all predicates from the calling module using listing/1. For example, ?- listing. lists clauses in the default user module and ?- lists:listing. lists the clauses in the module lists.
portray_clause(+Clause): Pretty print a clause. A clause should be specified as a term `<Head> :- <Body>'. Facts are represented as `<Head> :- true' or simply <Head>. Variables in the clause are written as A, B, ... . Singleton variables are written as _. See also portray_clause/2.
portray_clause(+Stream, +Clause): Pretty print a clause to Stream. See portray_clause/1 for details.