SublistUnbnd( l, ind )
Sublist of a list with possibly unbound entries.
The writing of this function was prompted by the fact that if l has
some unbound entries, l{ind} returns an error message instead of
doing what is expected (which is what this routine does).
gap> l := [ 1, , 2, , , 3 ];;
gap> SublistUnbnd( l, [ 1..4 ] );
[ 1,, 2 ]
If you use l{[ 1..4 ]}, you get an error message.
GAP 3.4.4