Normally `ln -sf path/to/target at/name` will create a symlink at
`at/name` that points to `path/to/target`. But if `at/name` already
exists and is a directory or a symlink to some other directory, then
this command will create a symlink at `at/name/target` pointing to
`path/to/target`. There is an ambiguity between 1st and 3rd form of
`ln` (please refer to `man ln` for the available invocation forms). It
can be disambiguated by using the `-T` flag to force the 1st form.
In our case, if `/etc/portage/patches` symlink already existed and was
pointing to `<coreos-overlay>/coreos/user-patches`, we ended up with a
useless symlink at `<coreos-overlay>/coreos/user-patches/user-patches`
pointing to `<coreos-overlay>/coreos/user-patches`.
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