code: boolean isModifiedBy(long txNum) {
  		internalLock.writeLock().lock();
  		try {
  			return modifiedBy.contains(txNum);
  		} finally {
  			internalLock.writeLock().unlock();
  		}
  	}
docs: Returns true if the buffer is dirty due to a modification by the
  specified transaction.
  
  @param txNum
  the id of the transaction
  @return true if the transaction modified the buffer