Dear fellow GAP users-

As part of my Master's research (in Computer Science), I have been studying
ways of performing substitutions of one term for another in a GAP expression.
Having had some experience with Maple, I was initially surprised to discover
that GAP provides no analogue of the "subs" function in Maple. The issue
arose because I had a polynomial in (Q(alpha))[x], within which I wanted to
substitute beta for alpha to obtain a new polynomial in (Q(beta))[x]. (alpha
and beta were algebraic numbers lying in different minimal inclusive fields.)
My immediate solution was to write a special-purpose substitution routine for
the task at hand, but I was left wondering about the absence of a more general
facility.

I think I now understand the reasons no such facility exists. In designing
some limited substitution facilities, however, I've envisioned some kinds of
substitutions which may well be feasible, but not actually useful to anyone.
So (getting to the point of this message at last !) I would like to hear from
other users of GAP, and other symbolic math systems, about this question.
What kinds of substitutions do you find useful in GAP (et al.) ? What kinds
of substitutions are possible but not helpful (or even meaningful) to you in
other systems ? Please describe cases of substitutions you would like to
perform in GAP, and the approach you've taken to handling them.

I greatly appreciate your time in reading this, and in replying if you choose
to do so. I get bombarded with email daily, so I certainly know the effort
involved in processing it all and still getting everything else done !

I'd be happy to summarize private responses to the Forum, and let you know
what comes out of this if you'd like.

Thanks
-Lewis McCarthy
for replies to this, please use: lmccarth@klingon.cs.umass.edu