this.value = pair.substring(stop, pair.length());
this.arg = arg; this.value = value;
  public ArgValue(String pair, String sep) {    // the pair must be split on sep to produce arg and value    if (pair == null || sep == null) {      // error!      throw new IllegalArgumentException("Can't parse with null Strings");    }        int start = pair.indexOf(sep);    if (start < 1) {      // error!      throw new IllegalArgumentException("Separator not found, or null Argument");    }        int stop = start + sep.length();    //this(pair.substring(0, start), pair.substring(stop, pair.length() - 1));    this.arg = pair.substring(0, start);    if (this.arg == null) {      throw new IllegalArgumentException("Need a non-null argument");    }    this.value = pair.substring(stop, pair.length());  }