my @tokens = get_tokens $string;

my sub parse_list {
   my @stack;
   push @stack, [ ];
   while ( @tokens ) {
      my $token = shift( @tokens ) {
           
      if ( $token eq "(" ) {
         my $sublist = [ ];
         push @{ $stack[ -1 ] }, $sublist;
         push @stack, $sublist;
      }
      elsif ( $token eq ")" ) {
         die( "Unexpected `)` ) if @stack == 1;
         pop( @stack );
      }
      else {
         push @{ $stack[ -1 ] }, $token;
      }
   }

   die( "Missing `)` ) if @stack > 1;
   return $stack[ 0 ];
}

my $list = parse_list();
die( "Unexpected `$tokens[0]`" ) if @tokens;