<<
fn mylexfn(context: &mut p_context_t, out_token_info: &mut p_token_info_t) -> usize {
    let result = p_lex(context, out_token_info);
    if result != P_SUCCESS {
        return result;
    }
    if out_token_info.token == TOKEN_lparen {
        /* Reentrant nested parse of the parenthesized sub-expression. */
        let inner_result = p_parse_inner_Start(context, &[TOKEN_rparen]);
        if inner_result != P_SUCCESS {
            return inner_result;
        }
        let value = p_result_Start(context);
        /* p_parse_inner rewound the input so ')' was not consumed; consume it. */
        let mut rparen_info = p_token_info_t::default();
        assert_eq!(P_SUCCESS, p_lex(context, &mut rparen_info));
        assert_eq!(TOKEN_rparen, rparen_info.token);
        out_token_info.token = TOKEN_num;
        out_token_info.pvalue = p_value(value);
    }
    P_SUCCESS
}
>>

ptype i64;

lex_fn mylexfn;

drop /\s+/;
token lparen /\(/;
token rparen /\)/;
token plus /\+/;
token num /\d+/ <<
  let mut v: i64 = 0;
  for c in match_text { v = v * 10 + (*c - b'0') as i64; }
  $$ = v;
>>

Start -> Expr << $$ = $1; >>
Expr -> num << $$ = $1; >>
Expr -> Expr plus num << $$ = $1 + $3; >>