array.c:635
static VALUE
rb_ary_rindex(ary, val)
    VALUE ary;
    VALUE val;
{
    long i = RARRAY(ary)->len;

    while (i--) {
	if (rb_equal(RARRAY(ary)->ptr[i], val))
	    return LONG2NUM(i);
    }
    return Qnil;
}
