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: multi.rs
    path of file: ./repos/parity-ethereum/ethcore/engines/validator-set/src
    
    the code of the file until where you have to start completion: // Copyright 2015-2020 Parity Technologies (UK) Ltd.
// This file is part of Parity Ethereum.

// Parity Ethereum is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Parity Ethereum is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Parity Ethereum.  If not, see <http://www.gnu.org/licenses/>.

/// Validator set changing at fork blocks.

use std::collections::BTreeMap;
use std::sync::Weak;

use common_types::{
	BlockNumber,
	header::Header,
	ids::BlockId,
	errors::EthcoreError,
	engines::machine::{Call, AuxiliaryData},
};
use client_traits::EngineClient;
use ethereum_types::{H256, Address};
use log::{debug, trace};
use parity_bytes::Bytes;
use parking_lot::RwLock;
use machine::Machine;

use super::{SystemCall, ValidatorSet};

type BlockNumberLookup = Box<dyn Fn(BlockId) -> Result<BlockNumber, String> + Send + Sync + 'static>;

pub struct Multi {
	sets: BTreeMap<BlockNumber, Box<dyn ValidatorSet>>,
	block_number: RwLock<BlockNumberLookup>,
}

impl Multi {
	pub fn new(set_map: BTreeMap<BlockN
    
	fn uses_current_set() {
		let tap = Arc::new(AccountProvider::transient_provider());
		let s0: Secret = keccak("0").into();
		let v0 = tap.insert_account(s0.clone(), &"".into()).unwrap();
		let v1 = tap.insert_account(keccak("1").into(), &"".into()).unwrap();
		let client = generate_dummy_client_with_spec(spec::new_validator_multi);
		client.engine().register_client(Arc::downgrade(&client) as _);

		// Make sure txs go through.
		client.miner().set_gas_range_target((1_000_000.into(), 1_000_000.into()));

		// Wrong signer for the first block.
		let signer = Box::new((tap.clone(), v1, "".into()));
		client.miner().set_author(miner::Author::Sealer(signer));
		client.transact(TransactionRequest::call(Default::default(), Default::default())).unwrap();
		EngineClient::update_sealing(&*client, ForceUpdateSealing::No);
		assert_eq!(client.chain_info().best_block_number, 0);
		// Right signer for the first block.
		let signer = Box::new((tap.clone(), v0, "".into()));
		client.miner().set_author(miner::Author::Sealer(signer));
		EngineClient::update_sealing(&*client, ForceUpdateSealing::No);
		assert_eq!(client.chain_info().best_block_number, 1);
		// This time v0 is wrong.
		client.transact(TransactionRequest::call(Default::default(), Default::default())).unwrap();
		EngineClient::update_sealing(&*client, ForceUpdateSealing::No);
		assert_eq!(client.chain_info().best_block_number, 1);
		let signer = Box::new((tap.clone(), v1, "".into()));
		client.miner().set_author(miner::Author::Sealer(signer));
		EngineClient::update_sealing(&*client, ForceUpdateSealing::No);
		assert_eq!(client.chain_info().best_block_number, 2);
		// v1 is still good.
		client.transact(TransactionRequest::call(Default::default(), Default::default())).unwrap();
		EngineClient::update_sealing(&*client, ForceUpdateSealing::No);
		assert_eq!(client.chain_info().best_block_number, 3);

		// Check syncing.
		let sync_client = generate_dummy_client_with_spec(spec::new_validator_multi);
		sync_client.engine().register_client(Arc::downgrade(&sync_client) as _);
		for i in 1..4 {
			sync_client.import_block(Unverified::from_rlp(client.block(BlockId::Number(i)).unwrap().into_inner()).unwrap()).unwrap();
		}
		sync_client.flush_queue();
		assert_eq!(sync_client.chain_info().best_block_number, 3);
	}