code: func (st *state) compactNumber() int {
  	if len(st.str) == 0 {
  		st.fail("missing index")
  	}
  	if st.str[0] == '_' {
  		st.advance(1)
  		return 0
  	} else if st.str[0] == 'n' {
  		st.fail("unexpected negative number")
  	}
  	n := st.number()
  	if len(st.str) == 0 || st.str[0] != '_' {
  		st.fail("missing underscore after number")
  	}
  	st.advance(1)
  	return n + 1
  }
docs: // <non-negative number> _ */