MissingValueContextException example

    // If there are any mappings that were not satisfied, throw an exception.     // This is a more severe problem than missing values, so check and throw     // this first.     if (!empty($mappings)) {
      throw new ContextException('Assigned contexts were not satisfied: ' . implode(',', array_keys($mappings)));
    }

    // If there are any required contexts without a value, throw an exception.     if ($missing_value) {
      throw new MissingValueContextException($missing_value);
    }
  }

}