copies: document hack for adding '' to set of dirs The root directory is not normally added to 'dirs' instances (although I think it should be). In copies.mergecopies, we call dirname() to get the directory of a path and then check for containment in the 'dirs' instances ('d1' and 'd2'). In order to easily handle files in the root directory, '/' is added to d1/d2. This results in the empty string being added to the sets, since what comes before the slash in '/' is an empty string. This seems less than obvious, so let's document it.