Availability:C-language interface function
bool PL_get_arg(size_t
index, term_t +t, term_t -a)If t is compound and index is between 1 and arity
(inclusive), assign a with a term reference to the argument.
Returns
FALSE if t is not a compound or index
is out of range (zero or higher than the arity of the compound). This
function never raises a Prolog exception.