# Call as: flatten([...])
sub flatten {
  my $aref=shift;
  die "not an array ref" unless ref($aref) eq 'ARRAY';
  @result=();
  foreach my $element (@$aref) {
    push @result,
        (ref($element) eq 'ARRAY' 
        ? flatten($element) 
        : $element);
  }
  @result;
}