feat(fc): prove FC-4 the fork-choice tree is acyclic - #44
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Store/Ancestry.lean states block ancestry relationally: ProperAncestor st a d holds when parent_root links from d reach a in one or more steps inside the store. Upstream has no standalone helper - the convention lives in the _checkpoint_is_ancestor walk and in the parent-slot ordering established at block insertion, stated here as WellFormed.parentSlotLt (leanEthereum/leanSpec#1176). properAncestor_slot_lt shows slots strictly decrease along every proper-ancestor step, so a self-ancestor cycle would put a block's slot strictly below itself: fork_choice_acyclic.
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Summary
Second FC proposition, building on the
Storemodel andStore.WellFormedfrom #43.Approach
Store/Ancestry.leanstates block ancestry relationally:ProperAncestor st a dholds whenparent_rootlinks fromdreachain one or more steps inside the store. Upstream has no standaloneis_proper_ancestorhelper — the convention lives in the_checkpoint_is_ancestorwalk (already modeled ascheckpointIsAncestor) and in the parent-slot ordering established at block insertion (on_blockruns the STF, which admits only strictly-future slots), stated asWellFormed.parentSlotLt(leanEthereum/leanSpec#1176).Theorems
ProperAncestor.descendant_block— the descendant end of every derivation is a stored blockgetBlock?_isSome_of_mem— a stored entry makes the lookup succeedproperAncestor_slot_lt— slots strictly decrease along proper ancestry on a well-formed storefork_choice_acyclic(FC-4) — no stored block is a proper ancestor of itself: a cycle would put the block's slot strictly below itselfCatalog: FC-4 checked (with a note that blocks are keyed pairs, so the statement quantifies over
p ∈ st.blocks), progress table 18/12/1.lake buildpasses with nosorry.