Unfortunately, I had to change the implementation, see 1eaf74d. The reason can be seen [in a small self-contained test](https://gist.github.com/krlmlr/6159962) I have prepared. This test fails, but works if line 20 is replaced by `f2e <- .expr_to_fun(inc())`. Unfortunately, the code in the test is an extract of what is happening for, e.g., `raply` or `rdply` -- and `rlply` if a generic worker function is used.