sub by_mp(;$) { my $map = shift; my $c = ref $map || $map;
   ...init...

    my $arg_lvh = $map->_by_mp($arg);

    if (ref $arg_lvh) {
      $arg_lvh->fs_mp($arg);
      return $arg_lvh;    #<----seems to return from here
    }

    my $devno = (stat $arg)[0];

    $arg_lvh = $map->by_devno($devno);
    return undef unless $arg_lvh;
    die P "by_mp: by_devno(%s) returned %s, of type \"%s\", not a ref\n",
          $devno, $arg_lvh, ref $arg_lvh unless ref $arg_lvh;
    $map->_by_mp($arg, $arg_lvh);
    $arg_lvh->fs_mp($arg);
    return $arg_lvh;
  }