@Override
    public String toString() {
        StringBuilder buffer = new StringBuilder();
        for (Iterator<Production> i = productions.iterator(); i.hasNext(); ) {
            buffer.append(i.next());
            if (i.hasNext())
                buffer.append(" ");
        }
        return buffer.toString();
    }
Returns an unmodifiable view of the productions in this
container-search/src/main/java/com/yahoo/prelude/semantics/rule/ProductionList.java