Nothing
eval.by <- function(f,expr) UseMethod("eval.by")
eval.by.default <- function(f,expr){
parent <- parent.frame()
expr <- substitute(expr)
vars <- all.vars(expr)
vars <- data.frame(sapply(vars,get,parent))
vars <- split(vars,f)
res <- lapply(vars,function(v)eval(expr,v,parent))
unsplit(res,f)
}
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