code: public void moveHead(int delta) {
  		assert !isRecycled() : "Attempt to use recycled bytebuf";
  		assert head + delta >= 0;
  		assert head + delta <= tail;
  		head += delta;
  	}
docs: Sets new value of {@link #head} by moving it by the given delta
  if this {@link ByteBuf} is not recycled.
  
  @param delta the value by which current {@link #head} will be moved.
  New {@link #head} must be bigger or equal to 0
  and smaller or equal to {@link #tail}