/*
 *  call-seq:
 *     term_doc_enum.doc -> doc_id
 *
 *  Move forward to the next document in the enumeration. Returns +true+ if
 *  there is another document or +false+ otherwise.
 */
static VALUE
frt_tde_next_position(VALUE self)
{
    TermDocEnum *tde = (TermDocEnum *)DATA_PTR(self);
    int pos;
    if (tde->next_position == NULL) {
        rb_raise(rb_eNotImpError, "to scan through positions you must create "
                 "the TermDocEnum with Index#term_positions method rather "
                 "than the Index#term_docs method");
    }
    pos = tde->next_position(tde);
    return pos >= 0 ? INT2FIX(pos) : Qnil;
}