public LookupIdentifier getNestedLookupIdentifier(final String newKey) {
        if (!isMapNested()) {
            throw new RuntimeException(LogUtil.message("Identifier {} is not nested", this));
        }
        return new LookupIdentifier(secondaryMapName, newKey, eventTime);
    }
@return The secondary map name which may itself be a nested map, e.g. "MAP2/MAP3"
stroom-pipeline/src/main/java/stroom/pipeline/refdata/LookupIdentifier.java