sS nameRelativeToPhysicalSubdirectory(File inner, File outer) ctex {
  if (inner == null || outer == null) null;
  S path1 = inner.getCanonicalPath();
  S path2 = addFileSep(outer.getCanonicalPath());
  new Matches m;
  if (startsWith(path1, path2, m))
    ret m.rest();
  else
    null;
}