[r-t] Extension

[Don Morrison]

On Fri, Jul 25, 2008 at 6:43 AM, Robin Woolley <robin at robinw.org.uk> wrote:
> In fact, the mechanical parts of the Decisions (e.g., (G)C2(b)(ii))
> could be given in one line. That would be fun, wouldn't it?

I would be most interested in seeing that. While presumably it would
not be a good replacement for more motivation filled presentations
were it the only one, as one of an ensemble of different ways of
describing the same thing it might help me finally to understand
what's really going on with this stuff.

I'd also be interested to see it presented as a pair of clear, unambiguous,
directly implementable algorithms in pseudo code or something like
that:

1) Given as input a method M at stage n, and a small, positive integer i,
produce as output a list of all valid extensions of M to stage n+2i.

2) Given as input a method M at stage n, and a small, positive integer
i less than floor(n/2), produce as output a list of all valid
contractions of M to stage n-2i.



-- 
Don Morrison <dfm at ringing.org>
"Any word can be either screamed or grunted, so if you have merely a
word written on paper you have to know not only its meaning but
something about its context before it can tell you whether to grunt
or to scream."             -- William Empson, _Seven Types of Ambiguity_