code: function(value)
  {
     // Note: A value is a reference to a subject if all of these hold true:
     // 1. It is an Object.
     // 2. It is has an @id key.
     // 3. It has only 1 key.
     return (value !== null &&
        value.constructor === Object &&
        '@id' in value &&
        Object.keys(value).length === 1);
  }
docs: Gets whether or not a value is a reference to a subject (or a subject with
  no properties).
  
  @param value the value to check.
  
  @return true if the value is a reference to a subject, false if not.