src/library/base/man/by.Rd has
\item{INDICES}{a factor or a list of factors, each of
length \code{nrow(x)}.}
\item{FUN}{a function to be applied to data frame
subsets of \code{x}.}
'x' should be 'data' in both.
Also, in
\details{
A data frame is split by row into data frames
subsetted by the values of one or more factors, and function
\code{FUN} is applied to each subset in term.
the last word was probably meant to be 'turn'.
-Deepayan
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