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Document weird code involving link trades of mew and celebi

This piece of code is frankly rather confusing, as it seemingly doesn't
serve much of a purpose. Documenting it here for future reference.

--- a/engine/link/link.asm

+++ b/engine/link/link.asm

@@ -1882,24 +1882,31 @@

 	call LoadTradeScreenBorder

 	call SetTradeRoomBGPals

 	farcall Link_WaitBGMap

-	ld b, $1

+

+; Check if either of the Pokémon sent was a Mew or Celebi, and send a different

+; byte depending on that. Presumably this would've been some prevention against

+; illicit trade machines, but it doesn't seem like a very effective one.

+; Removing this code breaks link compatibility with the vanilla gen2 games, but

+; has otherwise no consequence.

+	ld b, 1

 	pop af

 	ld c, a

 	cp MEW

-	jr z, .loop

+	jr z, .send_checkbyte

 	ld a, [wCurPartySpecies]

 	cp MEW

-	jr z, .loop

-	ld b, $2

+	jr z, .send_checkbyte

+	ld b, 2

 	ld a, c

 	cp CELEBI

-	jr z, .loop

+	jr z, .send_checkbyte

 	ld a, [wCurPartySpecies]

 	cp CELEBI

-	jr z, .loop

-	ld b, $0

+	jr z, .send_checkbyte

-.loop

+; Send the byte in a loop until the desired byte has been received.

+	ld b, 0

+.send_checkbyte

 	ld a, b

 	ld [wPlayerLinkAction], a

 	push bc

@@ -1913,7 +1920,7 @@

 	jr z, .save

 	ld a, [wOtherPlayerLinkAction]

 	cp b

-	jr nz, .loop

+	jr nz, .send_checkbyte

 .save

 	farcall SaveAfterLinkTrade