def METHOD_NAME(mining_chain):
    chain_a, chain_b = build(
        mining_chain,
        chain_split(
            (mine_block(extra_data=b"chain-a"), mine_block()),
            (mine_block(extra_data=b"chain-b"), mine_block(), mine_block()),
        ),
    )
    first_a = chain_a.get_canonical_block_by_number(1).header
    first_b = chain_b.get_canonical_block_by_number(1).header
    assert first_a.extra_data == b"chain-a"
    assert first_b.extra_data == b"chain-b"
    head_a = chain_a.get_canonical_head()
    assert head_a.block_number == 2
    head_b = chain_b.get_canonical_head()
    assert head_b.block_number == 3