code: @Override
    public <VALUEBASE, VALUE extends VALUEBASE, KEY extends Key<CoreMap, VALUEBASE>>
      VALUE set(Class<KEY> key, VALUE value) {
      
      if (immutableKeys.contains(key)) {
        throw new HashableCoreMapException("Attempt to change value " +
        		"of immutable field "+key.getSimpleName());
      }
      
      return super.set(key, value);
    }
docs: Sets the value associated with the given key; if the the key is one
  of the hashable keys, throws an exception.
  
  @throws HashableCoreMapException Attempting to set the value for an
  immutable, hashable key.