?- 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. lists clauses in the default user 
module and ?- lists:listing. lists the clauses in the 
module
lists.<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.