diff --git a/beacon_chain/spec/datatypes/altair.nim b/beacon_chain/spec/datatypes/altair.nim
index 4e2f82ff2..db51581ec 100644
--- a/beacon_chain/spec/datatypes/altair.nim
+++ b/beacon_chain/spec/datatypes/altair.nim
@@ -525,5 +525,5 @@ func shortLog*(v: SyncAggregate): auto =
 
 chronicles.formatIt SyncCommitteeMessage: shortLog(it)
 
-func hash*(x: LightClientUpdate): Hash =
-  hash(x.header.state_root.data)
+template hash*(x: LightClientUpdate): Hash =
+  hash(x.header)
diff --git a/beacon_chain/spec/datatypes/base.nim b/beacon_chain/spec/datatypes/base.nim
index f8d13c099..dfd841d4e 100644
--- a/beacon_chain/spec/datatypes/base.nim
+++ b/beacon_chain/spec/datatypes/base.nim
@@ -921,6 +921,9 @@ proc load*(
   else:
     some(validators[index.int].pubkey.loadValid())
 
+template hash*(header: BeaconBlockHeader): Hash =
+  hash(header.state_root)
+
 static:
   # Sanity checks - these types should be trivial enough to copy with memcpy
   doAssert supportsCopyMem(Validator)
diff --git a/beacon_chain/spec/helpers.nim b/beacon_chain/spec/helpers.nim
index 5adef81ad..64cdb6bce 100644
--- a/beacon_chain/spec/helpers.nim
+++ b/beacon_chain/spec/helpers.nim
@@ -236,4 +236,5 @@ func has_flag*(flags: ParticipationFlags, flag_index: int): bool =
 
 # https://github.com/ethereum/eth2.0-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#get_subtree_index
 func get_subtree_index*(idx: GeneralizedIndex): uint64 =
+  doAssert idx > 0
   uint64(idx mod (type(idx)(1) shl log2trunc(idx)))
diff --git a/beacon_chain/spec/light_client_sync.nim b/beacon_chain/spec/light_client_sync.nim
index 3544752e1..d0d45804b 100644
--- a/beacon_chain/spec/light_client_sync.nim
+++ b/beacon_chain/spec/light_client_sync.nim
@@ -1,16 +1,9 @@
 import
   std/sets,
-  stew/bitops2,
+  stew/[bitops2, objects],
   datatypes/altair,
   helpers
 
-func branchIsAllZeros(branch: openarray[Eth2Digest]): bool =
-  for node in branch:
-    if node != Eth2Digest():
-      return false
-
-  return true
-
 # https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#validate_light_client_update
 proc validate_light_client_update*(snapshot: LightClientSnapshot,
                                    update: LightClientUpdate,
@@ -27,8 +20,8 @@ proc validate_light_client_update*(snapshot: LightClientSnapshot,
 
   # Verify update header root is the finalized root of the finality header, if specified
   # TODO: Use a view type instead of `unsafeAddr`
-  let signed_header = if update.finality_header == BeaconBlockHeader():
-    if not branchIsAllZeros(update.finality_branch):
+  let signed_header = if update.finality_header.isZeroMemory:
+    if not update.finality_branch.isZeroMemory:
       return false
     unsafeAddr update.header
   else:
@@ -43,7 +36,7 @@ proc validate_light_client_update*(snapshot: LightClientSnapshot,
   # Verify update next sync committee if the update period incremented
   # TODO: Use a view type instead of `unsafeAddr`
   let sync_committee = if update_period == snapshot_period:
-    if not branchIsAllZeros(update.next_sync_committee_branch):
+    if not update.next_sync_committee_branch.isZeroMemory:
       return false
     unsafeAddr snapshot.current_sync_committee
   else:
@@ -55,13 +48,14 @@ proc validate_light_client_update*(snapshot: LightClientSnapshot,
       return false
     unsafeAddr snapshot.next_sync_committee
 
+  let sync_committee_participants_count = countOnes(update.sync_committee_bits)
   # Verify sync committee has sufficient participants
-  if countOnes(update.sync_committee_bits) < MIN_SYNC_COMMITTEE_PARTICIPANTS:
+  if sync_committee_participants_count < MIN_SYNC_COMMITTEE_PARTICIPANTS:
     return false
 
   # Verify sync committee aggregate signature
   # participant_pubkeys = [pubkey for (bit, pubkey) in zip(update.sync_committee_bits, sync_committee.pubkeys) if bit]
-  var participant_pubkeys: seq[ValidatorPubKey]
+  var participant_pubkeys = newSeqOfCap[ValidatorPubKey](sync_committee_participants_count)
   for idx, bit in update.sync_committee_bits:
     if bit:
       participant_pubkeys.add(sync_committee.pubkeys[idx])
@@ -71,16 +65,16 @@ proc validate_light_client_update*(snapshot: LightClientSnapshot,
 
   blsFastAggregateVerify(participant_pubkeys, signing_root.data, update.sync_committee_signature)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#apply_light_client_update
+# https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#apply_light_client_update
 proc apply_light_client_update(snapshot: var LightClientSnapshot, update: LightClientUpdate) =
   let snapshot_period = compute_epoch_at_slot(snapshot.header.slot) div EPOCHS_PER_SYNC_COMMITTEE_PERIOD
   let update_period = compute_epoch_at_slot(update.header.slot) div EPOCHS_PER_SYNC_COMMITTEE_PERIOD
   if update_period == snapshot_period + 1:
-      snapshot.current_sync_committee = snapshot.next_sync_committee
-      snapshot.next_sync_committee = update.next_sync_committee
+    snapshot.current_sync_committee = snapshot.next_sync_committee
+    snapshot.next_sync_committee = update.next_sync_committee
   snapshot.header = update.header
 
-# https://github.com/ethereum/eth2.0-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#process_light_client_update
+# https://github.com/ethereum/consensus-specs/blob/v1.1.0-beta.3/specs/altair/sync-protocol.md#process_light_client_update
 proc process_light_client_update(store: var LightClientStore,
                                  update: LightClientUpdate,
                                  current_slot: Slot,
@@ -92,7 +86,7 @@ proc process_light_client_update(store: var LightClientStore,
   var update_timeout = SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD
   let sync_committee_participants_count = countOnes(update.sync_committee_bits)
   if sync_committee_participants_count * 3 >= update.sync_committee_bits.len * 2 and
-     update.finality_header != BeaconBlockHeader():
+     not update.finality_header.isZeroMemory:
     # Apply update if (1) 2/3 quorum is reached and (2) we have a finality proof.
     # Note that (2) means that the current light client design needs finality.
     # It may be changed to re-organizable light client design. See the on-going issue eth2.0-specs#2182.
@@ -100,6 +94,12 @@ proc process_light_client_update(store: var LightClientStore,
     store.valid_updates.clear()
   elif current_slot > store.snapshot.header.slot + update_timeout:
     var best_update_participants = 0
+    # TODO:
+    #  Use a view type to avoid the copying when a new best update
+    #  is discovered.
+    #  Alterantively, we could have a `set.max` operation returning
+    #  the best item as a `lent` value. We would need an `OrderedSet`
+    #  type with support for a custom comparison operation.
     var best_update: LightClientUpdate
     for update in store.valid_updates:
       let update_participants = countOnes(update.sync_committee_bits)
@@ -110,4 +110,4 @@ proc process_light_client_update(store: var LightClientStore,
     # Forced best update when the update timeout has elapsed
     apply_light_client_update(store.snapshot, best_update)
     store.valid_updates.clear()
-  return true
+  true
diff --git a/beacon_chain/ssz/merkleization.nim b/beacon_chain/ssz/merkleization.nim
index 1d4b6a9d9..141a75ed5 100644
--- a/beacon_chain/ssz/merkleization.nim
+++ b/beacon_chain/ssz/merkleization.nim
@@ -32,6 +32,8 @@ func binaryTreeHeight*(totalElements: Limit): int =
   bitWidth nextPow2(uint64 totalElements)
 
 type
+  # TODO Figure out what would be the right type for this.
+  #      It probably fits in uint16 for all practical purposes.
   GeneralizedIndex* = uint32
 
   SszMerkleizerImpl = object
diff --git a/vendor/nim-stew b/vendor/nim-stew
index 3c91b8694..478cc6efd 160000
--- a/vendor/nim-stew
+++ b/vendor/nim-stew
@@ -1 +1 @@
-Subproject commit 3c91b8694e15137a81ec7db37c6c58194ec94a6a
+Subproject commit 478cc6efdefaabadf0666a3351fb959b78009bcc