code: func (i ItemType) precedence() int {
  	switch i {
  	case ItemLOR:
  		return 1
  	case ItemLAND, ItemLUnless:
  		return 2
  	case ItemEQL, ItemNEQ, ItemLTE, ItemLSS, ItemGTE, ItemGTR:
  		return 3
  	case ItemADD, ItemSUB:
  		return 4
  	case ItemMUL, ItemDIV, ItemMOD:
  		return 5
  	case ItemPOW:
  		return 6
  	default:
  		return LowestPrec
  	}
  }
docs: // Precedence returns the operator precedence of the binary
  // operator op. If op is not a binary operator, the result
  // is LowestPrec.