'return a string which is the relative part of the path
to the given \'relpath\'.'

def relto(self, relpath):

    if (not isinstance(relpath, (str, PathBase))):
        raise TypeError(('%r:    not    a    string    or    path    object' % (relpath,)))
    strrelpath = str(relpath)
    if (strrelpath and (strrelpath[(-1)] != self.sep)):
        strrelpath += self.sep
    strself = self.strpath
    if ((sys.platform == 'win32') or (getattr(os, '_name', None) == 'nt')):
        if os.path.normcase(strself).startswith(os.path.normcase(strrelpath)):
            return strself[len(strrelpath):]
    elif strself.startswith(strrelpath):
        return strself[len(strrelpath):]
    return ''
