code: func (p *Parser) parseOperator() (op Operator, err error) {
  	tok, lit := p.consume(KeyAndOperator)
  	switch tok {
  	// DoesNotExistToken shouldn't be here because it's a unary operator, not a binary operator
  	case InToken:
  		op = InOperator
  	case EqualsToken:
  		op = EqualsOperator
  	case DoubleEqualsToken:
  		op = DoubleEqualsOperator
  	case NotInToken:
  		op = NotInOperator
  	case NotEqualsToken:
  		op = NotEqualsOperator
  	default:
  		return "", fmt.Errorf("found '%s', expected: '=', '!=', '==', 'in', notin'", lit)
  	}
  	return op, nil
  }
docs: // parseOperator return operator and eventually matchType
  // matchType can be exact