code: func isNumber(s string) bool {
  	if len(s) == 0 {
  		return false
  	}
  	if !IsDigit(rune(s[0])) {
  		if len(s) < 2 || s[0] != '-' || !IsDigit(rune(s[1])) {
  			return false
  		}
  	}
  	return true
  }
docs: // isNumber is only used in eval.go