A safety check. If ... is not specified, R asks for
confirmation if removeOldCache is used interactively.
If confirmed interactively by a user or if
... contains argument named y and equal "y",
i.e. with a call removeOldCache(y = "y"),
the old cache files are deleted, i.e. the files for those
CachedResult objects that are no longer present in the
current environment. removeOldCache looks for the current
CachedResult objects also nested inside lists.