@Override
    public String toString() {
        StringBuilder ret = new StringBuilder();
        for (int i = 0, len = items.size(); i < len; ++i) {
            Item item = items.get(i);
            if (i != 0) {
                ret.append(" ").append(item.type.image).append(" ");
            }
            ret.append(item.exp.toString());
        }
        return ret.toString();
    }
Converts the internal list of operator-expression pairs into a corresponding combined grouping expression. When
this method returns there is no residue of the conversion, and this object can be reused.

@return The grouping expression corresponding to the pushed arithmetic operations.
container-search/src/main/java/com/yahoo/search/grouping/request/MathResolver.java