Set#merge

static VALUE
set_i_merge(int argc, VALUE *argv, VALUE set)
{
    if (rb_keyword_given_p()) {
        rb_raise(rb_eArgError, "no keywords accepted");
    }

    if (set_iterating_p(set)) {
        rb_raise(rb_eRuntimeError, "cannot add to set during iteration");
    }

    rb_check_frozen(set);

    int i;

    for (i=0; i < argc; i++) {
        set_merge_enum_into(set, argv[i]);
    }

    return set;
}