code: public void startNewKeyGroup(int keyGroupId) throws IOException {
  		if (isKeyGroupAlreadyStarted(keyGroupId)) {
  			throw new IOException("Key group " + keyGroupId + " already registered!");
  		}
  		keyGroupRangeOffsets.setKeyGroupOffset(keyGroupId, delegate.getPos());
  		currentKeyGroup = keyGroupId;
  	}
docs: User code can call this method to signal that it begins to write a new key group with the given key group id.
  This id must be within the {@link KeyGroupsList} provided by the stream. Each key-group can only be started once
  and is considered final/immutable as soon as this method is called again.