sub insert_array_elem {
   my ($ra, $elem, $index) = @_; # insert $elem before $ra->[$index]

   if ($index < 0) {
     # convert negative indexes to positive equiv.
     $index = @$ra + $index;
   }

   if ($index == 0) {
     # at the beginning
     unshift @$ra, $elem;
   }
   elsif ($index == @$ra) {
     # at the end
     push @$ra, $elem;
   }
   else {
     # insert between -- makes copy: bad for large arrays?
     @$ra = @$ra[0..$index-1], $elem, @$ra[$index..@$ra-1];
   }
}