code: func NewOperandEnumErr(pos int, expected ...string) error {
  
  	if len(expected) == 1 {
  		return NewOperandErr(pos, "must be %v", expected[0])
  	}
  
  	return NewOperandErr(pos, "must be one of {%v}", strings.Join(expected, ", "))
  }
docs: // NewOperandEnumErr returns an operand error indicating a value was wrong.