code: func compareSimpleVariables(a, b ast.Variable) (bool, error) {
  	if a.Type != b.Type {
  		return false, fmt.Errorf(
  			"won't compare items of different types %s and %s",
  			a.Type.Printable(), b.Type.Printable())
  	}
  	switch a.Type {
  	case ast.TypeString:
  		return a.Value.(string) == b.Value.(string), nil
  	default:
  		return false, fmt.Errorf(
  			"can't compare items of type %s",
  			a.Type.Printable())
  	}
  }
docs: // compare two variables of the same type, i.e. non complex one, such as TypeList or TypeMap