Correct the unionWith fusion wording

The fusion is a constant-factor change, not the elimination of the These
intermediate it was described as. Map.union still produces a per-key These
and the inner These is still built transiently for shared keys; what the
fusion removes is one of the two outer Map.map passes over the post-union
map, plus the wrap-then-remap of single-side currencies. Adjust the
changelog and drop Note [Single-pass unionWith] accordingly; the merge
structure is plain from the code.

For IntersectMBO/plutus-private#2243.

Author: Unisay

plutus-ledger-api/changelog.d/20260527_131134_yuriy.lazaryev_issue_2243_fused_unionwith.md

@@ -1,6 +1,6 @@
 ### Changed
 
-- Fused `unionWith` in `PlutusLedgerApi.V1.Data.Value` into two outer passes (`Map.union` + a single `Map.map`), down from the previous three. The new implementation no longer materialises the intermediate `Map TokenName (These Integer Integer)` stage that the discarded `unionVal` helper used to produce; `These` wrapping survives only at the outer `CurrencySymbol` level, where it is collapsed in place.
+- Fused `unionWith` in `PlutusLedgerApi.V1.Data.Value` into two outer passes (`Map.union` + a single `Map.map`), down from the previous three. The discarded `unionVal` helper returned a full `Map CurrencySymbol (Map TokenName (These Integer Integer))` that `unionWith` then re-traversed; folding it in removes that second outer walk, and a currency present on only one side now has its token map mapped once rather than wrapped and then re-mapped. The `Map.union` lookup-and-merge core is unchanged, so this is a constant-factor improvement, not an algorithmic one.
 
 - Fused `checkBinRel` along the same shape, walking the outer `Map.union` result with `Map.all` and applying the relation directly against `0` on whichever side is missing. The walk short-circuits on the first failing pair, giving `geq` / `leq` / `gt` / `lt` early exit on inputs that violate the relation near the start of their key set.
 

plutus-ledger-api/src/PlutusLedgerApi/V1/Data/Value.hs

@@ -402,13 +402,6 @@ assetClassValueOf :: Value -> AssetClass -> Integer
 assetClassValueOf v (AssetClass (c, t)) = valueOf v c t
 {-# INLINEABLE assetClassValueOf #-}
 
-{- Note [Single-pass unionWith]
-'Map.union' tags each currency symbol with a 'These' recording which side(s)
-hold it. 'fuseInners' consumes that tag in one outer 'Map.map': against an
-implicit @0@ on the absent side, or — when both sides hold the symbol — over
-the inner @Map.union innerL innerR@. The merge therefore touches each level
-once and never materialises a @Map TokenName (These Integer Integer)@. -}
-
 {-| Combine two 'Value' maps with the argument function.
 Assumes the well-definedness of the two maps. -}
 unionWith :: (Integer -> Integer -> Integer) -> Value -> Value -> Value
@@ -454,9 +447,8 @@ isZero (Value xs) = Map.all (Map.all (\i -> 0 == i)) xs
 {-| Check whether a binary relation holds for value pairs of two 'Value' maps,
   supplying 0 where a key is only present in one of them.
 
-Shares the structure of 'unionWith' (see Note [Single-pass unionWith]), with
-'Map.all' in place of 'Map.map': the walk short-circuits on the first pair
-that fails @f@, applying the relation against @0@ on whichever side is absent. -}
+Walks the outer 'Map.union' with 'Map.all', short-circuiting on the first pair
+that fails @f@. -}
 checkBinRel :: (Integer -> Integer -> Bool) -> Value -> Value -> Bool
 checkBinRel f (Value mapL) (Value mapR) =
   Map.all checkInners (Map.union mapL mapR)