@Override
    public void optimize(RankingExpression expression, ContextIndex context, OptimizationReport report) {
        if ( ! isEnabled()) return;

        this.report = report;
        expression.setRoot(findAndOptimize(expression.getRoot()));
        report.note("GBDT forest optimization done");
    }
Optimizes sums of GBDTNodes by replacing them by a single GBDTForestNode

@param expression the expression to destructively optimize
@param context    a fast lookup context created from the given expression
@param report     the optimization report to which actions of this is logged
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/evaluation/gbdtoptimization/GBDTForestOptimizer.java