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Old 12-31-2013, 12:48 PM
zenobia43 zenobia43 is offline
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Join Date: Mar 2012
Posts: 175

OK I am getting the picture now. Is it the same logic if it were ?:- May is either Parrot or Finch. I guess that wouldn't be the same logic would it?
If Aurora was the Parrot, then the logic would be the same as "May is either Aurora or finch". However, in general, the logic is different.

And getting back to the old sortie of; Of A and B one is C and the other is D. Where A is either C or D and B is either C or D.
If A is a colour can we only mark negatives against C and D as negative colours? and vice versa for B, C, D?
This is the double exclusive OR clue. If A is a category with colours (like in the skydiver backstory), and B category is the year the jumper started jumping, then check the B category for colour exclusions (a.k.a. negative colours in your post).

C and D can only be the colours allowable by the explicit colour of A in the clue, and any colours that B can be at the point the clue is processed.

That is, C and D (the right side of the clue), can only be the colours found in the A and B options (the left side of the clue).

We know by convention that B cannot be the colour, A, but look for other colour exclusions for B. If you find that B cannot be a certain colour (other than A), then neither C nor D can be that colour.

One of the key conditions is that A and B are different categories.

You look in the A category for excluded B options, and you look in the B category for excluded A options. Move those exclusions to the C and D categories.

Then look in the C and D categories (assuming they are different categories) the same way. Look in the C category for excluded D options, and look in the D category for excluded C options. Move any exclusions found to the A and B categories.

I found it useful to practice this with grid paper putting the AB and CD intersections in various places in the solution grid and putting a variety of Xs and Os in the row and column of the intersection.

After a while, you can process these single and double exclusive OR clues visually.
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