@Override
    public void freeze() {
        if (isFrozen()) return;
        resolvePlaceholders();
        section.freeze();
        sources=getSources();
        super.freeze();
    }
Returns an unmodifiable set of all the sources this template <i>may</i> include (depending on choice resolution).
If the template allows (somewhere) the "any" source (*), Source.any will be in the set returned.
This operation is fast on frozen page templates (i.e at execution time).
container-search/src/main/java/com/yahoo/search/pagetemplates/PageTemplate.java