“该进程已分叉...”在R中使用tm包时出错

Question

我在tm package中安装了R来进行一些文本挖掘分析。创建语料库后，我想使用tm_map()函数，它会抛出以下错误消息：

The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().Break on 

有人知道为什么会出现此消息？

以下是更多澄清的代码：

> require(tm)
Lade nötiges Paket: tm
> a <-Corpus(VectorSource(chi2014_df$text))
> a
A corpus with 70 text documents
> a <-tm_map(a,tolower)
The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().
Break on 

我在OSX 10.7.5上使用R 3.0.1 GUI 1.61 Snow Leopard build（6492）

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