private void verifyBool(String id, String val) {
        if ("true".equalsIgnoreCase(val) || "false".equalsIgnoreCase(val)) {
            verifyBool(id);
        } else {
            defFail(id, val, "bool", null);
        }
    }
Returns the root ConfigDefinition, might be this.
config/src/main/java/com/yahoo/vespa/config/ConfigDefinition.java