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: uncles_verifier.rs
    path of file: ./repos/ckb/verification/contextual/src
    
    the code of the file until where you have to start completion: use ckb_chain_spec::consensus::Consensus;
use ckb_error::Error;
use ckb_types::{
    core::{BlockNumber, BlockView, EpochExt, HeaderView},
    packed::Byte32,
};
use ckb_verification::{PowError, UnclesError};
use std::collections::{HashMap, HashSet};

pub trait UncleProvider {
    fn double_inclusion(&self, hash: &Byte32) -> bool;

    fn consensus(&self) -> &Consensus;

    fn epoch(&self) -> &EpochExt;

    fn descendant(&self, uncle: &HeaderView) -> bool;
}

#[derive(Clone)]
pub struct UnclesVerifier<'a, P> {
    provider: P,
    block: &'a BlockView,
}

// A block B1 is considered to be the uncle of another block B2 if all of the following conditions are met:
// (1) they are in the same epoch, sharing the same difficulty;
// (2) height(B2) > height(B1);
// (3) B1's parent is either B2's ancestor or embedded in B2 or its ancestors as an uncle;
// and (4) B2 is the first block in its chain to refer to B1.
impl<'a, P> UnclesVerifier<'a, P>
where
    P: UncleProvider,
{
    pub fn new(provider: P, block: &'a BlockView) -> Self {
        UnclesVerifier { provider, block }
    }

    // -  uncles_num
    // -  depth
    // -  uncle not in main chain
    // -  uncle duplicate
    pub fn verify(&self) -> Result<(), Error> {
        let uncles_count = self.block.data().uncles().len() as u32;

        // if self.block.uncles is empty, return
        if uncles_count == 0 {
            return Ok(());
        }

        // if block is genesis, which is expected with zero uncles, return error
        if self.block.is_genesis() {
            return Err(UnclesError::OverCount {
                max: 0,
                actual: uncles_count,
            }
            .into());
        }

        // verify uncles length =< max_uncles_num
        let max_uncles_num = self.provider.consensus().max_uncles_num() as u32;
        if uncles_count > max_uncles_num {
            return Err(UnclesError::OverCount {
                max: max_uncles_num,
                actual: uncles_count,
            }
            .into());
        }

        let mut included: HashMap<Byte32, BlockNumber> = HashMap::default();
        for uncle in self.block.uncles().into_iter() {
            if uncle.compact_target() != self.provider.epoch().compact_target() {
                return Err(UnclesError::InvalidTarget.into());
            }

            if self.provider.epoch().number() != uncle.epoch().number() {
                return Err((UnclesError::InvalidDifficultyEpoch).into());
            }

            if uncle.number() >= self.block.number() {
                return Err((UnclesError::InvalidNumber).into());
            }

            let embedded_descendant = included
                .get(&uncle.data().header().raw().parent_hash())
                .map(|number| (number + 1) == uncle.number())
                .unwrap_or(false);

            if !(embedded_descendant || self.provider.descendant(&uncle.header())) {
    
    
    pub fn verify(&self) -> Result<(), Error> {
        let uncles_count = self.block.data().uncles().len() as u32;

        // if self.block.uncles is empty, return
        if uncles_count == 0 {
            return Ok(());
        }

        // if block is genesis, which is expected with zero uncles, return error
        if self.block.is_genesis() {
            return Err(UnclesError::OverCount {
                max: 0,
                actual: uncles_count,
            }
            .into());
        }

        // verify uncles length =< max_uncles_num
        let max_uncles_num = self.provider.consensus().max_uncles_num() as u32;
        if uncles_count > max_uncles_num {
            return Err(UnclesError::OverCount {
                max: max_uncles_num,
                actual: uncles_count,
            }
            .into());
        }

        let mut included: HashMap<Byte32, BlockNumber> = HashMap::default();
        for uncle in self.block.uncles().into_iter() {
            if uncle.compact_target() != self.provider.epoch().compact_target() {
                return Err(UnclesError::InvalidTarget.into());
            }

            if self.provider.epoch().number() != uncle.epoch().number() {
                return Err((UnclesError::InvalidDifficultyEpoch).into());
            }

            if uncle.number() >= self.block.number() {
                return Err((UnclesError::InvalidNumber).into());
            }

            let embedded_descendant = included
                .get(&uncle.data().header().raw().parent_hash())
                .map(|number| (number + 1) == uncle.number())
                .unwrap_or(false);

            if !(embedded_descendant || self.provider.descendant(&uncle.header())) {
                return Err((UnclesError::DescendantLimit).into());
            }

            if included.contains_key(&uncle.hash()) {
                return Err((UnclesError::Duplicate(uncle.hash())).into());
            }

            if self.provider.double_inclusion(&uncle.hash()) {
                return Err((UnclesError::DoubleInclusion(uncle.hash())).into());
            }

            if uncle.data().proposals().len()
                > self.provider.consensus().max_block_proposals_limit() as usize
            {
                return Err((UnclesError::ExceededMaximumProposalsLimit).into());
            }

            if uncle.proposals_hash() != uncle.data().as_reader().calc_proposals_hash() {
                return Err((UnclesError::ProposalsHash).into());
            }

            let mut seen = HashSet::with_capacity(uncle.data().proposals().len());
            if !uncle
                .data()
                .proposals()
                .into_iter()
                .all(|id| seen.insert(id))
            {
                return Err((UnclesError::ProposalDuplicate).into());
            }

            if !self
                .provider
                .consensus()
                .pow_engine()
                .verify(&uncle.data().header())
            {
                return Err((PowError::InvalidNonce).into());
            }

            included.insert(uncle.hash(), uncle.number());
        }

        Ok(())
    }