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[Stephen Weeks]

> The only place where I make use of Void is in MLton_eq, where two voids
> are always equal.  If you're confident enough about never propagating back
> voids to the codegens, we could change the MachineOutput.Operand.t type to
> reflect that.

I am confident (I even put an assert in), but I don't want to get rid of void
for now, since there is a place in backend where I would have to create an
Operand.t option instead of an Operand.t, and I don't wanna pay that cost.