<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _foldername(self, additionalpath=""):
 """Dot decorate a folder name."""

        if not self._foldername_cache.get(additionalpath):
            fn = joinpath(self.base, self.folder, additionalpath) \
                if not self.is_subfolder \
                else joinpath(self.base, ".%s" % self.folder, additionalpath)
            self._foldername_cache[additionalpath] = fn
        return self._foldername_cache[additionalpath]