`IUSE`, `KEYWORDS` or `LICENSE` variable. With `%` and `?`, it is done
against values specified in `package.*` files.
+Please denote that for keywords, `*` and `**` arguments have special
+meaning and will not be parsed as patterns. If you need to perform
+pattern matching there, please use `?*` instead.
In addition to that, the argument can be preceded by a namespace
specifier in the form `ns::`, where `ns` can be one of `use`, `kw` or