blue wrote:Here is a diagram showing ...

The missing "bit of detail", in his description, are how the rest of the diagram -- the D,E,F,U,V,W truths, and thier links into each other and the candidates in the A,B,R,S,T truths -- conspire to make it impossible to have any of these situations occuring in a solution:He leaves TGT1 out, above. I'm not sure whether he's assuming that its presence is implied, or saying that it doesn't need to be there.

- TGT1 and one of {Rx,Ry} and one of {Sx,Sy} true
- TGT1 and one of {Rx,Ry} and one of {Tx,Ty} true
- TGT1 and one of {Sx,Sy} and one of {Tx,Ty} true

Hi blue,

That diagram is very close to the one produced by Allan Barker himself, and your conclusion reflects the difficulty at that time to explain why it worked.

In Allan general construction, he had in theory to find the rank 0 area of the diagram but the diagram has 6 triple points (link type). The best clue he gave was that these triple point where linked (2 groups of three).

It is just to have a simpler explanation that the exocet logic came.