Nope, we completely agree on this point! What I was trying to illustrate was B does not need to care whether or not A signed with ACP: in all cases, a 3rd party observer would need to strip B's inputs from the transaction before adding their own. More specifically, if A insecurely signs with ACP, B can still safely sign the transaction with ALL. Either the transaction will go through, or B's inputs will be removed from the transaction. A should never sign with ACP because doing so puts A's funds at risk of being stolen or burned.
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