Dear GAP-Forum,

Walter Carlip wrote:

Today after a long computation (I let it run overnight) I
obtained a particular sought-after subgroup of a general linear
group over a finite field, say GL(3,7). I needed to study
properties of the center of this group G, and carfully defined
Z := Centre(G). Much to my surprise everything broke down, since
Z is evidently an "internal" function used in conjunction with
finite fields.

After making such a blunder, is there any way to recover
the original definition of the internal function Z?

Not really. The only thing you can do at this point is printing the results
--- if possible --- to a file and starting anew. If you fear, that you are
prone to do such an error again (though probably the loss of a day's work is
scaring enough to remember not to use a variable 'Z'), you can do the
following:
Create or edit Your '.gaprc' file (on UNIX machines this file is in Your home
directory but will only be listed by 'ls -a', so do not wonder if it seems
to have disappeared) and add the following line:

TheOriginalDefinitionOfZ:=Z;

This will copy the original definition of Z to a variable
(which is presumably weirdly enough named to be misused). As it is in the
'.gaprc' file, the assignement is executed every time you start GAP, so you
don't have to do anything special afterwards. If you
accidentally redefine Z, you can get back the original definition by an
assignement the other way round.

Keep in mind that redefinements of E, EA-EZ, CF and GF will lead to
similarly catastrophical situations. However all GAP functions and variables
(ignoring for a moment 'time' which is more internal) provided by the
library start with upper case letters. So making all your own variables to
be lower case will always force you to be on the safe side.

Hope this helps,

Alexander