docs: static String getDirectoryFromSystemProp(final String prop) {
      final String dir = System.getProperty(prop);
      String err = null;
      if (dir == null) {
        err = "' is not set.";
      } else if (dir.isEmpty()) {
        err = "' is empty.";
      } else if (dir.charAt(dir.length() - 1) != '/') {  // Screw Windows.
        err = "' is not terminated with `/'.";
      }
      if (err != null) {
        throw new IllegalStateException("System property `" + prop + err);
      }
      return dir;
    }