@Override
    public void freeze() {
        if (isFrozen()) return;
        super.freeze();
        placeholderIds=Collections.unmodifiableList(placeholderIds);
        values=Collections.unmodifiableList(values);
    }
Returns the values (the "from" of the mapping) of this as a live reference which can be modified unless
this is frozen. Note that each single choice of values within this is also a list of values. This is
the inner list.
container-search/src/main/java/com/yahoo/search/pagetemplates/model/MapChoice.java