code: protected void addSerializableClass(Class<?> serializable, Class<?> mixin) {
          safeMapper.addMixIn(serializable, mixin);
          fullMapper.addMixIn(serializable, mixin);
      }
docs: This protected method allows a subclass to add to the mappers a class type that can be
  serialized using mixin class.
  
  @param serializable The type of class that can be serialized using its toString() method.
  @param mixin The type of class that can be used to serialized the serializable class.