Not to belabor this...but here's a paraphrase of one of their drill questions:

Diagram the following statement: Y is examined before X and Z are examined. There are 6 examinations:

-................ -.....-.....-.....-........-

(~X,~Z)...................~Y..~Y

What the hell?! Now they've flipped. They DO recognize that X and Z could occur together in the second slot, and so just eliminate the possibility of them occurring first, but now Y for some reason cannot occur 2nd last. But that presumes X and Z cannot occur simultaneously in the last slot!

That's it...I'm throwing away this book.

The above reasoning is off... even if X and Z could not occur together in the second slot, you can't know for sure that neither of them is going to be there, so they (powerscore) are not having you make a note of it. If you were to make a note of it, best you could do is ~Xv~Z (with v='or'). That's not much information, and would likely confuse most people when they were working out the problems.

But...

Okay, I hate to put it this way, but you may be confusing yourself. Know that I don't have access to the Powerscore materials you're talking about, and so I can't give you direct references, also keep in mind that even when it's not stated explicitly in one of the rules/conditions, the set up will often tell you or give some reason that there can only be one element per 'slot.' I suspect that there is something in the examples you are giving that would indicate this.

Going one possibility at a time.

A:

6 slots. P after both Q and R (only one element per slot):

We can deduce that there is no P in slots 1 or 2.

We can deduce that there is no Q and no R in slot 6.

We don't really know anything about slot 5, as P, Q or R could still be there individually.

B:

6 slots. P after both Q and R (more than one element possible per slot):

We can deduce there is no P in slot 1. (unlike case A above, we don't really know anything about slot 2, if Q and R are together in slot 1, P could go there.)

We can deduce that there is no Q and no R in slot 6.

We

**STILL** don't really know anything about slot 5 as P, Q or R or (Q and R) could still go there.

C:

6 slots. Y before X and Z. (One element possible per slot.)

We can deduce that neither X nor Z can go in slot 1: that would give no place for Y.

We don't really know anything about slot 2. Y or X or Z could each be there.

We can deduce that Y can not go into slot 5 or 6, as there must be room for X and Z in two separate subsequent slots.

D:

6 Slots. Y before X and Z. (More than one element possible per slot.)

We can deduce that neither X nor Z can go into slot 1, as above.

We

**still** don't know anything about slot 2. No matter how many elements can go there, Y or X or Z or (X and Z) could go there.

We can deduce that Y can not go into slot 6. Here, we don't know about slot 5 though. Y could go there, with X and Z in slot 6.