protected boolean hasOpenChoicepoint(RuleEvaluation e) {
        return false;
    }
Returns whether there is an <i>open choice</i> in this or any of its subconditions.
Returns false by default, must be overriden by conditions which may generate
choices open accross multiple calls to matches, or contain such conditions.
container-search/src/main/java/com/yahoo/prelude/semantics/rule/Condition.java