private void addIndexField(String indexName,SDField field) {
        List<SDField> fields = indexToFields.get(indexName);
        if (fields == null) {
            fields = new java.util.ArrayList<>();
            indexToFields.put(indexName, fields);
        }
        fields.add(field);
    }
A map from index names to a List of fields going to that index
config-model/src/main/java/com/yahoo/schema/processing/MultifieldIndexHarmonizer.java