'Use me if C{path} might have slashes in it, but you know they\'re safe.
@param path: A relative path (ie, a path not starting with C{"/"})
which will be interpreted as a child or descendant of this path.
@type path: L{bytes} or L{unicode}
@return: The child path.
@rtype: L{FilePath} with a mode equal to the type of C{path}.'

def preauthChild(self, path):

    ourPath = self._getPathAsSameTypeAs(path)
    newpath = abspath(joinpath(ourPath, normpath(path)))
    if (not newpath.startswith(ourPath)):
        raise InsecurePath(('%s    is    not    a    child    of    %s' % (newpath, ourPath)))
    return self.clonePath(newpath)
