diff --git a/mysticeti/validator.nim b/mysticeti/validator.nim
index b779e12..50c2f89 100644
--- a/mysticeti/validator.nim
+++ b/mysticeti/validator.nim
@@ -7,10 +7,10 @@ type
   Validator*[Signing, Hashing] = ref object
     identity: Identity[Signing]
     committee: Committee[Signing]
-    round: Round[Signing, Hashing]
+    first, last: Round[Signing, Hashing]
   Round[Signing, Hashing] = ref object
     number: uint64
-    previous: ?Round[Signing, Hashing]
+    previous, next: ?Round[Signing, Hashing]
     slots: Table[Identifier[Signing], ProposerSlot[Signing, Hashing]]
   ProposerSlot[Signing, Hashing] = object
     proposal: Block[Signing, Hashing]
@@ -24,7 +24,7 @@ type
 
 func new*(T: type Validator; identity: Identity, committee: Committee): T =
   let round = Round[T.Signing, T.Hashing](number: 0)
-  T(identity: identity, committee: committee, round: round)
+  T(identity: identity, committee: committee, first: round, last: round)
 
 func new*(_: type Round, number: uint64, previous: Round): auto =
   Round(number: number, previous: some previous)
@@ -36,20 +36,22 @@ func identifier*(validator: Validator): auto =
   validator.identity.identifier
 
 func round*(validator: Validator): uint64 =
-  validator.round.number
+  validator.last.number
 
 func wave(validator: Validator): auto =
   # A wave consists of 3 rounds: proposing -> voting -> certifying
-  type Round = typeof(validator.round)
-  let certifying = validator.round
+  type Round = typeof(validator.last)
+  let certifying = validator.last
   if voting =? certifying.previous:
     if proposing =? voting.previous:
       return some (proposing, voting, certifying)
   none (Round, Round, Round)
 
 func nextRound*(validator: Validator) =
-  let previous = validator.round
-  validator.round = Round.new(previous.number + 1, previous)
+  let previous = validator.last
+  let next = Round.new(previous.number + 1, previous)
+  validator.last = next
+  previous.next = some next
 
 func hasParent(blck: Block, round: uint64, author: Identifier): bool =
   for parent in blck.parents:
@@ -58,7 +60,7 @@ func hasParent(blck: Block, round: uint64, author: Identifier): bool =
   false
 
 func updateSkipped(validator: Validator, supporter: Block) =
-  if previous =? validator.round.previous:
+  if previous =? validator.last.previous:
     for (id, slot) in previous.slots.mpairs:
       if not supporter.hasParent(previous.number, id):
         slot.skippedBy += validator.committee.stake(supporter.author)
@@ -80,28 +82,28 @@ func updateCertified(validator: Validator, certificate: Block) =
       proposerSlot.status = ProposalStatus.toCommit
 
 proc propose*(validator: Validator, transactions: seq[Transaction]): auto =
-  assert validator.identifier notin validator.round.slots
+  assert validator.identifier notin validator.last.slots
   var parents: seq[BlockId[Validator.Signing, Validator.Hashing]]
-  if previous =? validator.round.previous:
+  if previous =? validator.last.previous:
     for id in previous.slots.keys:
       parents.add(previous.slots[id].proposal.id)
   let blck = Block.new(
     author = validator.identifier,
-    round = validator.round.number,
+    round = validator.last.number,
     parents = parents,
     transactions = transactions
   )
-  validator.round.slots[validator.identifier] = ProposerSlot.init(blck)
+  validator.last.slots[validator.identifier] = ProposerSlot.init(blck)
   validator.updateCertified(blck)
   validator.identity.sign(blck)
 
 func receive*(validator: Validator, signed: SignedBlock) =
-  validator.round.slots[signed.blck.author] = ProposerSlot.init(signed.blck)
+  validator.last.slots[signed.blck.author] = ProposerSlot.init(signed.blck)
   validator.updateSkipped(signed.blck)
   validator.updateCertified(signed.blck)
 
 func round(validator: Validator, number: uint64): auto =
-  var round = validator.round
+  var round = validator.last
   while round.number > number and previous =? round.previous:
     round = previous
   if round.number == number: