code: MemoEntry getMemo(String production, int position) {
  	Map<String, MemoEntry> map = memo.get(position);
  	if (map == null)
  	    return null;
  	MemoEntry memo = map.get(production);
  	if (memo == null)
  	    return null;
  	return memo;
      }
docs: This method returns the current memoization element from the buffer. This
  element is unprocessed. If the element is not set (null) a NONE element is
  put into the buffer and returned.
  
  @param production
  @param position
  @return