Validates the product equation.
    * Validates the feature order
    * Validates the product spec (mandatory functional features)
    :param poi: optional product of interest

def validate_product_equation(poi=None):
    """
    Validates the product equation.
    * Validates the feature order
    * Validates the product spec (mandatory functional features)
    :param poi: optional product of interest
    """
    from . import utils
    from . import validators

    container_dir, product_name = tasks.get_poi_tuple(poi=poi)
    feature_list = utils.get_features_from_equation(container_dir, product_name)
    ordering_constraints = utils.get_feature_order_constraints(container_dir)
    spec_path = utils.get_feature_ide_paths(container_dir, product_name).product_spec_path

    print('*** Starting product.equation validation')

    # --------------------------------------------------------
    # Validate the feature order
    print('\tChecking feature order')

    feature_order_validator = validators.FeatureOrderValidator(feature_list, ordering_constraints)
    feature_order_validator.check_order()

    if feature_order_validator.has_errors():
        print('\t\txxx ERROR in your product.equation feature order xxx')
        for error in feature_order_validator.get_violations():
            print('\t\t\t', error[1])
    else:
        print('\t\tOK')

    # --------------------------------------------------------
    # Validate the functional product specification
    print('\tChecking functional product spec')

    if not os.path.exists(spec_path):

        print(
            '\t\tSkipped - No product spec exists.\n'
            '\t\tYou may create a product spec if you want to ensure that\n'
            '\t\trequired functional features are represented in the product equation\n'
            '\t\t=> Create spec file featuremodel/productline/<container>/product_spec.json'
        )
        return

    spec_validator = validators.ProductSpecValidator(spec_path, product_name, feature_list)
    if not spec_validator.is_valid():
        if spec_validator.get_errors_mandatory():
            print('\t\tERROR: The following feature are missing', spec_validator.get_errors_mandatory())
        if spec_validator.get_errors_never():
            print('\t\tERROR: The following feature are not allowed', spec_validator.get_errors_never())
    else:
        print('\t\tOK')

    if feature_order_validator.has_errors() or spec_validator.has_errors():
        sys.exit(1)