# 0 --> identical element
        if ( $which_list == 0 ) {
            die "Can't happen" if @indices1 != @indices2;
            foreach my $i ( 0 .. $#indices1 ) {
                my ( $item1, @offsets ) = @{ $list1_aref->[ $indices1[
+$i] ] };
                my ( $item2, $offset  ) = @{ $list2_aref->[ $indices2[
+$i] ] };
                die unless $item1 eq $item2;
                push @offsets, $offset;
                push @merged, [ $item1, @offsets ];
            }
        }

        # 1 --> list 1 has an element here
        if ( $which_list & 1 ) {
            foreach ( @{ $list1_aref }[ @indices1 ] ) {
                my ( $item1, @offsets ) = @{ $_ };
                push @offsets, undef;
                push @merged, [ $item1, @offsets ];
            }
        }

        # 2 --> list 2 has an element here
        if ( $which_list & 2 ) {
            foreach ( @{ $list2_aref }[ @indices2 ] ) {
                my ( $item2, $offset ) = @{ $_ };
                my @offsets = map { undef() } 1 .. $offset_width;
                push @offsets, $offset;
                push @merged, [ $item2, @offsets ];
            }
        }
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