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tcoratgerclaude
tcoratger
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claude
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perf(fork-choice): prune votes on finalized-orphaned branches (#1148)
Pruning kept any vote whose head slot was strictly above the finalized slot. A vote whose head sits above the finalized slot but on a sibling branch the finalized block orphaned was retained and iterated forever, even though the LMD-GHOST walk anchored at the justified root can never reach it, so it adds zero weight. Equivocating votes on dead high-slot branches survived until finalization passed their head slot, leaking retention pressure. Add a finalized-ancestry guard to the prune: a vote survives only when its head is both above the finalized slot and a descendant of the finalized block. The justified root is always a descendant of the finalized root, so any vote able to credit a block on the canonical subtree is itself a descendant of the finalized block and is kept. Only provably-dead votes are additionally dropped; head, justified, and finalized are identical to before for every input. Add a fork_choice vector: a counted vote whose head sits above the finalized slot on a branch forked below the finalized block is pruned, while head, justified, and finalized stay on the canonical chain. Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Lines changed: 190 additions & 7 deletions

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‎src/lean_spec/spec/forks/lstar/fork_choice.py‎

Lines changed: 24 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -142,35 +142,52 @@ def prune_stale_attestation_data(self, store: LstarStore) -> LstarStore:
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Drop attestation data whose head can no longer influence fork choice.
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Fork choice only ever descends from the latest finalized block.
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A vote whose head sits at or below the finalized slot cannot name a descendant of it.
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Such a vote carries no fork-choice weight.
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Dropping it never changes the chosen chain.
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A vote carries no fork-choice weight, and is dropped, when either holds:
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- Its head sits at or below the finalized slot.
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- Its head is not a descendant of the finalized block.
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The first head is no later than the finalized block.
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The second head is on a sibling branch the finalized block orphaned.
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Neither head lies under the finalized block.
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So neither credits any block the forward walk from the justified root can reach.
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Dropping such a vote never changes the chosen chain.
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The same cutoff prunes all three attestation-keyed pools:
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- Per-validator attestation signatures.
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- Pending aggregated proofs not yet counted.
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- Aggregated proofs already counted toward fork choice.
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"""
155-
# Keep only entries whose attested head sits strictly above the finalized slot.
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# An entry survives only when its head still lives under the finalized block.
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#
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# The slot guard rejects heads at or below the finalized slot cheaply.
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# The ancestry guard then rejects heads on a finalized-orphaned sibling branch.
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# Such heads sit above the finalized slot yet never descend from the finalized block.
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# Every pool shares the same vote key, so one staleness test filters all three.
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def head_lives_under_finalized(attestation_data: AttestationData) -> bool:
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head = attestation_data.head
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return head.slot > store.latest_finalized.slot and self._checkpoint_is_ancestor(
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store, store.latest_finalized, head
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)
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return store.model_copy(
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update={
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"attestation_signatures": {
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attestation_data: signatures
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for attestation_data, signatures in store.attestation_signatures.items()
163-
if attestation_data.head.slot > store.latest_finalized.slot
180+
if head_lives_under_finalized(attestation_data)
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},
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"latest_new_aggregated_payloads": {
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attestation_data: proofs
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for attestation_data, proofs in store.latest_new_aggregated_payloads.items()
168-
if attestation_data.head.slot > store.latest_finalized.slot
185+
if head_lives_under_finalized(attestation_data)
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},
170187
"latest_known_aggregated_payloads": {
171188
attestation_data: proofs
172189
for attestation_data, proofs in store.latest_known_aggregated_payloads.items()
173-
if attestation_data.head.slot > store.latest_finalized.slot
190+
if head_lives_under_finalized(attestation_data)
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},
175192
}
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)

‎tests/consensus/lstar/fork_choice/test_prune_finalized_orphaned_branch.py‎

Lines changed: 166 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,166 @@
1+
"""Fork Choice: pruning drops votes on a finalized-orphaned branch."""
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import pytest
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from consensus_testing import (
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AggregatedAttestationSpec,
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BlockSpec,
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BlockStep,
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ForkChoiceTestFiller,
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GossipAggregatedAttestationStep,
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StoreChecks,
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generate_pre_state,
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)
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from lean_spec.spec.forks import Slot, ValidatorIndex
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pytestmark = pytest.mark.valid_until("Lstar")
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def test_finalization_prunes_vote_on_orphaned_branch(
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fork_choice_test: ForkChoiceTestFiller,
21+
) -> None:
22+
"""
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Finalization prunes a counted vote whose head sits above the finalized slot but
24+
on a branch the finalized block orphaned, while head, justified, and finalized
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stay exactly where the canonical chain puts them.
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Given
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-----
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- 8 validators; a slot needs 6 votes (2/3) to be justified.
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- the chain:
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genesis(0) -> block_1(1)
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- block_2(2) -> block_3(3) -> block_4(4)
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- orph_2(2) -> orph_3(3) -> orph_4(4)
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- block_2 carries V0..V5 targeting block_1, justifying slot 1.
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- block_3 carries V0..V5 targeting block_2, justifying slot 2 and finalizing slot 1.
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- the orphaned branch forks off block_1, below the eventual finalized slot 2.
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- a counted aggregate from V6 targets orph_4 at slot 4, above the finalized slot.
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- that vote's head orph_4 is not a descendant of block_2.
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When
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----
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- block_4 carries V0..V5 targeting block_3, justifying slot 3 and finalizing slot 2.
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Then
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----
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- head stays on block_4, on the canonical chain.
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- justified advances to slot 3 on block_3.
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- finalized advances to slot 2 on block_2.
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- the only counted target slot is 3, the one canonical vote whose head outlives slot 2.
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- the pending pool holds no target slots.
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- the orphaned vote targeting slot 4 is pruned despite its head outliving the finalized slot.
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"""
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fork_choice_test(
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anchor_state=generate_pre_state(num_validators=8),
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steps=[
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BlockStep(
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block=BlockSpec(slot=Slot(1), parent_label="genesis", label="block_1"),
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checks=StoreChecks(head_slot=Slot(1)),
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),
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BlockStep(
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block=BlockSpec(
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slot=Slot(2),
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parent_label="block_1",
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label="block_2",
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attestations=[
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AggregatedAttestationSpec(
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validator_indices=[ValidatorIndex(i) for i in range(6)],
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slot=Slot(2),
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target_slot=Slot(1),
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target_root_label="block_1",
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),
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],
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),
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checks=StoreChecks(
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head_slot=Slot(2),
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head_root_label="block_2",
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latest_justified_slot=Slot(1),
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latest_finalized_slot=Slot(0),
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),
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),
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BlockStep(
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block=BlockSpec(
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slot=Slot(3),
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parent_label="block_2",
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label="block_3",
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attestations=[
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AggregatedAttestationSpec(
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validator_indices=[ValidatorIndex(i) for i in range(6)],
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slot=Slot(3),
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target_slot=Slot(2),
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target_root_label="block_2",
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source_slot=Slot(1),
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source_root_label="block_1",
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),
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],
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),
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checks=StoreChecks(
98+
head_slot=Slot(3),
99+
head_root_label="block_3",
100+
latest_justified_slot=Slot(2),
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latest_finalized_slot=Slot(1),
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),
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),
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BlockStep(
105+
block=BlockSpec(slot=Slot(2), parent_label="block_1", label="orph_2"),
106+
checks=StoreChecks(head_slot=Slot(3), head_root_label="block_3"),
107+
),
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BlockStep(
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block=BlockSpec(slot=Slot(3), parent_label="orph_2", label="orph_3"),
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checks=StoreChecks(head_slot=Slot(3), head_root_label="block_3"),
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),
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BlockStep(
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block=BlockSpec(slot=Slot(4), parent_label="orph_3", label="orph_4"),
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checks=StoreChecks(
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head_slot=Slot(3),
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head_root_label="block_3",
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latest_justified_slot=Slot(2),
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latest_finalized_slot=Slot(1),
119+
),
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),
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GossipAggregatedAttestationStep(
122+
attestation=AggregatedAttestationSpec(
123+
validator_indices=[ValidatorIndex(6)],
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slot=Slot(4),
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target_slot=Slot(4),
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target_root_label="orph_4",
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head_root_label="orph_4",
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head_slot=Slot(4),
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source_slot=Slot(0),
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source_root_label="genesis",
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),
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checks=StoreChecks(
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head_slot=Slot(3),
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head_root_label="block_3",
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latest_new_aggregated_target_slots=[Slot(4)],
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),
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),
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BlockStep(
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block=BlockSpec(
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slot=Slot(4),
141+
parent_label="block_3",
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label="block_4",
143+
attestations=[
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AggregatedAttestationSpec(
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validator_indices=[ValidatorIndex(i) for i in range(6)],
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slot=Slot(4),
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target_slot=Slot(3),
148+
target_root_label="block_3",
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source_slot=Slot(2),
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source_root_label="block_2",
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),
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],
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),
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checks=StoreChecks(
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head_slot=Slot(4),
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head_root_label="block_4",
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latest_justified_slot=Slot(3),
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latest_justified_root_label="block_3",
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latest_finalized_slot=Slot(2),
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latest_finalized_root_label="block_2",
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latest_known_aggregated_target_slots=[Slot(3)],
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latest_new_aggregated_target_slots=[],
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),
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),
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],
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)

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