public static ModelReference valueOf(String s) {
        String[] parts = s.split(" ");
        if (parts.length == 1)
            return resolved(Path.of(s));
        else if (parts.length == 3)
            return unresolved(parts[0].equals("\"\"") ? Optional.empty() : Optional.of(parts[0]),
                              parts[1].equals("\"\"") ? Optional.empty() : Optional.of(new UrlReference(parts[1])),
                              parts[2].equals("\"\"") ? Optional.empty() : Optional.of(new FileReference(parts[2])));
        else
            throw new IllegalArgumentException("Unexpected model reference string '" + s + "'");
    }
Creates a model reference which is either a single string with no spaces if resolved, or if unresolved
a three-part string on the form <code>modelId url path</code>, where
each of the elements is either a value not containing space, or empty represented by "".
config-lib/src/main/java/com/yahoo/config/ModelReference.java