public PlanType transformExpressionsDown(PartialFunction<Expression, Expression> rule) {
        ExpressionTransformeDown transform = new ExpressionTransformeDown(rule);
        List<Object> newArgs = mapToList(args(), transform);
        return transform.changed ? withNewArgs(newArgs) : (PlanType) this;
    }
Runs [[transformDown]] with `rule` on all expressions present in this query operator.

@param rule the rule to be applied to every expression in this operator.
indexr-query-opt/src/main/java/io/indexr/query/plan/QueryPlan.java