如何从外部函数调用将分组变量传递给ddply？

I have a bunch of functions that are wrappers for ddply and other plyr functions. I occasionally need to change the grouping variables that I use in those functions and I'd like to keep them in a global variable so I only have to change one thing to effect the behavior of all the functions. Here's what I'm trying.I have a bunch of functions that are wrappers f