presto-main/src/main/java/io/prestosql/sql/planner/iterative/IterativeOptimizer.java/IterativeOptimizer/exploreChildren
class IterativeOptimizer:    private boolean exploreChildren(int group, Context context)
    {
        boolean progress = false;

        PlanNode expression = context.memo.getNode(group);
        for (PlanNode child : expression.getSources()) {
            checkState(child instanceof GroupReference, "Expected child to be a group reference. Found: " + child.getClass().getName());

            if (exploreGroup(((GroupReference) child).getGroupId(), context)) {
                progress = true;
            }
        }

        return progress;
    }