ast_for_trailer(struct compiling *c, const node *n, expr_ty left_expr)
{
    /* trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
       subscriptlist: subscript (',' subscript)* [',']
       subscript: '.' '.' '.' | test | [test] ':' [test] [sliceop]
     */
    const node *n_copy = n;
    REQ(n, trailer);
    if (TYPE(CHILD(n, 0)) == LPAR) {
        if (NCH(n) == 2)
            return Call(left_expr, NULL, NULL, LINENO(n), n->n_col_offset,
                        n->n_end_lineno, n->n_end_col_offset, c->c_arena);
        else
            return ast_for_call(c, CHILD(n, 1), left_expr, CHILD(n, 0), CHILD(n, 2));
    }
    else if (TYPE(CHILD(n, 0)) == DOT) {
        PyObject *attr_id = NEW_IDENTIFIER(CHILD(n, 1));
        if (!attr_id)
            return NULL;
        return Attribute(left_expr, attr_id, Load,
                         LINENO(n), n->n_col_offset,
                         n->n_end_lineno, n->n_end_col_offset, c->c_arena);
    }
    else {
        REQ(CHILD(n, 0), LSQB);
        REQ(CHILD(n, 2), RSQB);
        n = CHILD(n, 1);
        if (NCH(n) == 1) {
            slice_ty slc = ast_for_slice(c, CHILD(n, 0));
            if (!slc)
                return NULL;
            return Subscript(left_expr, slc, Load, LINENO(n), n->n_col_offset,
                             n_copy->n_end_lineno, n_copy->n_end_col_offset,
                             c->c_arena);
        }
        else {
            /* The grammar is ambiguous here. The ambiguity is resolved
               by treating the sequence as a tuple literal if there are
               no slice features.
            */
            Py_ssize_t j;
            slice_ty slc;
            expr_ty e;
            int simple = 1;
            asdl_seq *slices, *elts;
            slices = _Py_asdl_seq_new((NCH(n) + 1) / 2, c->c_arena);
            if (!slices)
                return NULL;
            for (j = 0; j < NCH(n); j += 2) {
                slc = ast_for_slice(c, CHILD(n, j));
                if (!slc)
                    return NULL;
                if (slc->kind != Index_kind)
                    simple = 0;
                asdl_seq_SET(slices, j / 2, slc);
            }
            if (!simple) {
                return Subscript(left_expr, ExtSlice(slices, c->c_arena),
                                 Load, LINENO(n), n->n_col_offset,
                                 n_copy->n_end_lineno, n_copy->n_end_col_offset, c->c_arena);
            }
            /* extract Index values and put them in a Tuple */
            elts = _Py_asdl_seq_new(asdl_seq_LEN(slices), c->c_arena);
            if (!elts)
                return NULL;
            for (j = 0; j < asdl_seq_LEN(slices); ++j) {
                slc = (slice_ty)asdl_seq_GET(slices, j);
                assert(slc->kind == Index_kind  && slc->v.Index.value);
                asdl_seq_SET(elts, j, slc->v.Index.value);
            }
            e = Tuple(elts, Load, LINENO(n), n->n_col_offset,
                      n->n_end_lineno, n->n_end_col_offset, c->c_arena);
            if (!e)
                return NULL;
            return Subscript(left_expr, Index(e, c->c_arena),
                             Load, LINENO(n), n->n_col_offset,
                             n_copy->n_end_lineno, n_copy->n_end_col_offset, c->c_arena);
        }
    }
}
Unknown