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