I'm using GAP 3.4.4. In GAP Manual Section 7.65 GroupId, several
predefined group names are given. Some of them (e.g. SL(d,q), GL(d,q) )
can be used to initialize a variable to the required group. Thus,
G := SL(2,7);
is sensible but
G := PSL(2,7);
is not. Where is a list of the group names which can be used to
initialize a variable? What to do if one needs an unavailable group?