@Override
    protected String conditionToRankingExpression() {
        StringBuilder b = new StringBuilder("in [");
        for (Iterator<Object> i = setValues.iterator(); i.hasNext(); ) {
            Object value = i.next();
            if (value instanceof String)
                b.append("\"").append(value).append("\"");
            else if (value instanceof Integer)
                b.append(value);
            else
                throw new RuntimeException("Excepted a string or integer in a set membership test, not a " +
                                           value.getClass() + ": " + value);

            if (i.hasNext())
                b.append(",");
        }
        b.append("]");
        return b.toString();
    }
Returns the unmodifiable set of values to check
searchlib/src/main/java/com/yahoo/searchlib/treenet/rule/SetMembershipCondition.java