Function build_subblock 

async fn build_subblock(
    transactions: Arc<Mutex<IndexMap<TxHash, Arc<Recovered<TempoTxEnvelope>>>>>,
    node: TempoFullNode,
    parent_hash: BlockHash,
    num_validators: usize,
    signer: PrivateKey,
    fee_recipient: Address,
    timeout: Duration,
) -> RecoveredSubBlock

Builds a subblock from candidate transactions we’ve collected so far.

This will include as many valid transactions as possible within the given timeout.