Actually addition is reversible, as long as you do it inline. The inverse of `a += b` is `a -= b`.

An example of an action that's not reversible is masking: `a &= b`. But if you dig deep enough into the physics, you find that these kinds of irreversible actions are actually always implemented in a way analogous to the following:

    // a &= b
    acquire zero'd register z from environment
    z ^= a & b
    z ^= a
    a ^= z
    z ^= a
    discard z into environment

Which can be reversed if you're able to get the discarded `z` register back.