[lolc]

The transaction sequence in your example is not possible. A transaction output can only be used once as an input. To me, the operation would rather look like these three transactions:

    100 Satoshi from A1
        2 Satoshi to A2
        98 Satoshi to Ax

    2 Satoshi from A2
        1 Satoshi to A3
        1 Satoshi to A4

    1 Satoshi from A3
    1 Satoshi from A4
        2 Satoshi to A5

Edit: Sorry I referred to "wallets" in my previous comment. This allows a scenario where the Satoshis are kept in separate transactions all the time. But I'm not interested in those scenarios. I'm talking about scenarios with multiple inputs and outputs. Where you can't say which of the inputs went which way. In this transaction:

    1 Satoshi from A1
    1 Satoshi from A2
        1 Satoshi to A3
        1 Satoshi to A4

You can't tell which Satoshi went where.

[g45y45]

Yeah, I will 'change' my position on this. Satoshis in a single UTXO that are split are fungible - in your scenario you cannot identify the specific sat: either a1->a3, or a1->a4. This is kind of like 'forward fungibility'. You can however trace the value in satoshi's back from A4 to their coinbase at birth. At best, we introduce a UTXO taint percentage from previous inputs. Its this taint from other, external inputs that break backwards fungibility (due to taint).