tinker more with the script

The program is now in an interesting position, where it finds the following
"ambiguous" position:

    . . . . . . . . . . . . .
     . . . . . . . . . . . . a
      b . . . . . . . . . # . .
       . . . . . . . # # # c . .
        . . . . . d # e e . . . .
         . . . . # d # . . f . . .
          . . . . # . # . . . . . .
           . . . g g . . . h . . i .
            . . . . . # . . . j . . .
             . . . . . . . . . # . # .
              . . . . . . . . . # . k .
               . . . . . . . . . . . . .
                . . . . . . . . . . . . .

    Ambiguous position: Array.new("a b cefhij d g k", "a b cefh d g ij k")
      in method reify at src/gen/m-CORE.setting:7828
      in method reify at src/gen/m-CORE.setting:7664
      in method gimme at src/gen/m-CORE.setting:8176
      in method sink at src/gen/m-CORE.setting:8635
      in block  at find-amb-positions:233
      in method reify at src/gen/m-CORE.setting:7828
      in method reify at src/gen/m-CORE.setting:7664
      in method gimme at src/gen/m-CORE.setting:8176
      in method sink at src/gen/m-CORE.setting:8635
      in block  at find-amb-positions:222

Two points on that:

- There's something funky going on with c ~ f and e ~ f. Since they share a
  link, this link should by rights be invalidated for whatever connection we
  don't make. (So I guess this *is* an ambiguous position, but not the one
  that the program flags up.)

- The ambiguity that *was* flagged up is clearly uninteresting, given that
  the only difference is whether we combine `cefh` and `ij` or not. Given the
  choice (to consider them combined or not) we would always go with "combined",
  and so it isn't much of an ambiguity. In fact, there's an induced partial
  ordering here (something like "y is more combined than x"), and interesting
  ambiguities are those where x and y are incomparable. (Such as `cf` and `ef`
  above.)