Regarding the SS bits in the return value of GetSlot for unexpanded slots, I added the early return so that they always remain zero. I thought I had changed this already but apparently had not submitted it yet 
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Two reasons for this;
One, to avoid confusing the reader needlessly. For the careful reader it can be a bit unexpected and warrants extra explanation, which I also thought about adding, but elaborating on that detail is not the point of the source code example. The question can be avoided by simply not having the situation in the first place.
Two, setting these bits to zero for unexpanded slots normalizes the value, that is, if you compare two normalized slot IDs then the values will be equal when the slot is equal. Whereas if the value is not normalized, an identity comparison is more complicated.
I also more precisely documented the return value to contain zeroes in bits 4-6. Even though for slot IDs these bits are irrelevant, your own code can use them for some purposes, in which case it can be useful to know that they can be assumed to be zero.
p.s. The previous suggested change of just adding a ret p was insufficient, since the routine should not modify hl.
