arg/3

Availability:built-in

[ISO]arg(?Arg, +Term, ?Value)

Term should be instantiated to a term, Arg to an integer between 1 and the arity of Term. Value is unified with the Arg-th argument of Term. Arg may also be unbound. In this case Value will be unified with the successive arguments of the term. On successful unification, Arg is unified with the argument number. Backtracking yields alternative solutions.^{117The instantiation
pattern (-, +, ?) is an extension to ‘standard’ Prolog. Some
systems provide genarg/3 that covers this pattern.} The predicate arg/3 fails silently if Arg = 0 or Arg > arity and raises the exception domain_error(not_less_than_zero, Arg) if Arg < 0.