Saved

The subexpressions saved to the cache, in the order their instructions were emitted. A
subexpression saved where a jump may pass over it has no value in its slot after the jump,
so once the code that may be passed over is compiled it is Forget(System.Int32)-ed back to
unsaved, and is computed again where it is next met: a slot is read only where every path
to the read has written it, as Substitute(System.Numerics.Complex[]) relies on.