@Override
    public void resolveLevel(int level) {
        super.resolveLevel(level);
        if (level < 1) {
            throw new IllegalArgumentException("Expression '" + this + "' not applicable for " +
                                               GroupingOperation.getLevelDesc(level) + ".");
        }
        if (exp != null) {
            exp.resolveLevel(level - 1);
        }
    }
Returns the expression that this node aggregates on.

@return The expression.
container-search/src/main/java/com/yahoo/search/grouping/request/AggregatorNode.java