public void addLegalUnarySwitch(String s, String explanation) {
        if (legalBinarySwitches.containsKey(s)) {
            throw new IllegalArgumentException(s +" already added as a binary switch");
        }
        legalUnarySwitches.put(s, explanation);
    }
Add a legal unary switch such as "-d"
vespajlib/src/main/java/com/yahoo/system/CommandLineParser.java