private Map<String, String> asModifiable(Map<String, String> map) {
        return new HashMap<>(map);
    }
A config map coming from the Vespa ConfigInstance is immutable while CustomAnalyzer builders
mutates the map to mark that a param was consumed. Immutable maps can't be mutated!
To overcome this conflict we can wrap the ConfigInstance map in a new HashMap.
lucene-linguistics/src/main/java/com/yahoo/language/lucene/AnalyzerFactory.java