You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: combine_operators.rs
    path of file: ./repos/FnckSQL/src/optimizer/rule/normalization
    
    the code of the file until where you have to start completion: use crate::errors::DatabaseError;
use crate::expression::{BinaryOperator, ScalarExpression};
use crate::optimizer::core::pattern::{Pattern, PatternChildrenPredicate};
use crate::optimizer::core::rule::{MatchPattern, NormalizationRule};
use crate::optimizer::heuristic::graph::{HepGraph, HepNodeId};
use crate::optimizer::rule::normalization::is_subset_exprs;
use crate::planner::operator::Operator;
use crate::types::LogicalType;
use lazy_static::lazy_static;
use std::collections::HashSet;

lazy_static! {
    static ref COLLAPSE_PROJECT_RULE: Pattern = {
        Pattern {
            predicate: |op| matches!(op, Operator::Project(_)),
            children: PatternChildrenPredicate::Predicate(vec![Pattern {
                predicate: |op| matches!(op, Operator::Project(_)),
                children: PatternChildrenPredicate::None,
            }]),
        }
    };
    static ref COMBINE_FILTERS_RULE: Pattern = {
        Pattern {
            predicate: |
    
    fn apply(&self, node_id: HepNodeId, graph: &mut HepGraph) -> Result<(), DatabaseError> {
        if let Operator::Aggregate(op) = graph.operator(node_id).clone() {
            // if it is an aggregation operator containing agg_call
            if !op.agg_calls.is_empty() {
                return Ok(());
            }

            if let Some(Operator::Aggregate(child_op)) = graph
                .eldest_child_at(node_id)
                .map(|child_id| graph.operator_mut(child_id))
            {
                if op.groupby_exprs.len() != child_op.groupby_exprs.len() {
                    return Ok(());
                }
                let mut expr_set = HashSet::new();

                for expr in op.groupby_exprs.iter() {
                    expr_set.insert(expr);
                }
                for expr in child_op.groupby_exprs.iter() {
                    expr_set.remove(expr);
                }
                if expr_set.is_empty() {
                    graph.remove_node(node_id, false);
                }
            }
        }

        Ok(())
    }