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