parameter_list
$$ = (parameter_list){ .first = $3, .identifier = true };
$$ = (parameter_list){ .used = $3.used };
parameter_list y_parameter_list;
$$ = (parameter_list){ .used = $4.used };
$$ = (parameter_list){ .vararg = true };
$$ = (parameter_list){ .first = $1 };
add_function(sym_t *decl, parameter_list params)
sym_t *add_function(sym_t *, parameter_list);