Sets package repositories.
useRepos(repos=NULL, where=c("before", "after", "replace"), unique=TRUE, fallback=TRUE,
...)A character string specifying how to add them to the
current set of repositories.
If TRUE, only unique repositories are set.
If TRUE, any remaining non-specified repository value
of format '...@' (e.g. '@CRAN@') than could not be recovered by
other means, will be assigned to a pre-defined known value, if possible.
If so, then an informative warning is given.
Not used.
Returns a list with element 'repos' reflecting options("repos")
as the options where prior to calling this function.
withRepos().