Derek Holt writes in his e-mail message of 13-Nov-92:

A user in Oxford (Jane Bamblett) has pointed out to me that the function
RestrictedPerm( <perm>, <list> )
fails when <perm> is the identity. This seems undesirable to me.

Indeed it does. A quick fix is to add the following three lines to the
file '<gap-dir>/lib/permutat.g'.

RestrictedPerm := function( g, D )
    local   res, d;

# check the arguments
if not IsPerm( g )  then
    Error("<g> must be a permutation");
elif not IsList( D )  then
    Error("<D> must be a list");
fi;

+     # handle the identity
+     if g = ()  then return ();  fi;
+
      # compute the restricted permutation <res>
      res := [ 1 .. Maximum( LargestMovedPointPerm(g), Maximum(D) ) ];
      for d  in D  do
          if not d^g in D  then
              Error("<D> must be invariant under the action of <g>");
          fi;
          res[d] := d^g;
      od;

# return the restricted permutation <res>
return PermList( res );
end;

Martin.

-- .- .-. - .. -.  .-.. --- ...- . ...  .- -. -. .. -.- .-
Martin Sch"onert,   Martin.Schoenert@Math.RWTH-Aachen.DE,  +49 241 804551
Lehrstuhl D f"ur Mathematik, Templergraben 64, RWTH, D 51 Aachen, Germany