code: def preauthChild(self, path):
          
          newpath = abspath(joinpath(self.path, normpath(path)))
          if not newpath.startswith(self.path):
              raise InsecurePath("%s is not a child of %s" % (newpath, self.path))
          return self.clonePath(newpath)
docs: Use me if `path' might have slashes in it, but you know they're safe.
  
          (NOT slashes at the beginning. It still needs to be a _child_).