Add more comments that order of transaction is important

hydra-node/src/Hydra/HeadLogic.hs

@@ -374,6 +374,8 @@ onOpenNetworkReqTx env ledger st ttl tx =
     RequestedSnapshot{} -> True
     SeenSnapshot{} -> True
 
+  -- NOTE: Order of transactions is important here. See also
+  -- 'pruneTransactions'.
   localTxs' = localTxs <> [tx]
 
 -- | Process a snapshot request ('ReqSn') from party.
@@ -514,7 +516,10 @@ onOpenNetworkReqSn env ledger st otherParty sv sn requestedTxIds mDecommitTx =
       Right u -> cont u
 
   pruneTransactions utxo = do
-    -- NOTE: Using foldl' is important to apply transacations in the right order.
+    -- NOTE: Using foldl' is important to apply transacations in the correct
+    -- order. That is, left-associative as new transactions are first validated
+    -- and then appended to `localTxs` (when aggregating
+    -- 'TransactionAppliedToLocalUTxO').
     foldl' go ([], utxo) localTxs
    where
     go (txs, u) tx =
@@ -1209,7 +1214,9 @@ aggregate st = \case
             { coordinatedHeadState =
                 coordinatedHeadState
                   { localUTxO = newLocalUTxO
-                  , localTxs = localTxs <> [tx]
+                  , -- NOTE: Order of transactions is important here. See also
+                    -- 'pruneTransactions'.
+                    localTxs = localTxs <> [tx]
                   }
             }
        where

hydra-node/src/Hydra/HeadLogic/State.hs

@@ -139,7 +139,9 @@ data CoordinatedHeadState tx = CoordinatedHeadState
   -- ^ The latest UTxO resulting from applying 'localTxs' to
   -- 'confirmedSnapshot'. Spec: L̂
   , localTxs :: [tx]
-  -- ^ List of transactions applied locally and pending inclusion in a snapshot. Spec: T̂
+  -- ^ List of transactions applied locally and pending inclusion in a snapshot.
+  -- Ordering in this list is important as transactions are added in order of
+  -- application. Spec: T̂
   , allTxs :: Map.Map (TxIdType tx) tx
   -- ^ Map containing all the transactions ever seen by this node and not yet
   -- included in a snapshot. Spec: Tall