Continuation of previous volume in TOC

[PaulRambags]

As Dedicon I want to be able to configure a special entry that should appear in the TOC when a volume break occurs in the middle of a section.
For instance, suppose Dotify decides to apply a volume break in Chapter 1, the TOC of the second volume should contain "Continuation of Chapter 1" and contents of the second chapter should start with "Contination of Chapter 1".

Notes:

1. The pipeline has level1 .. level6 and Dotify has only sequences and blocks.
2. First the OBFL interface must be changed.

Changes in OBFL:

• New elements toc-text and toc-text-resumed
• The content model of toc-entry will change to:
  toc-text [1], toc-text-resumed [0 or 1], toc-entry [0 or more]
• The content model of toc-text and toc-text-resumed is:
  text(), leader, marker, br, evaluate, page-number, span, style [0 or more]
• toc-sequence will have a new attribute use-toc-text-resumed [optional]
  One of true of false. The default is false.
  This allows, for instance, to have "Continuation of chapter ..." entries in all volume range TOCs, while the document range TOC does not have such entries.

Semantics:

• Every toc-entry refers to a block through attribute ref-id. If Dotify decides to apply a volume break such that the block is split accross several volumes, then toc-text-resumed is applied in all those volumes except the first one.
• The contents of all split volumes except the first one will start with toc-text-resumed. This functionality is similar to any-resumed in a volume-transition.
• Textual content resides at the toc-text element instead of at the the toc-entry element.

Remarks on the changes in OBFL:

• The change in content model of toc-entry is not backward compatible. The idea is to make the current content model deprecated, so that toc-entry can still contain these elements for a while: text(), leader, marker, br, evaluate, page-number, span, style.
• Structure and content of TOC entries are clearly separated, which is nice.
• It can happen that a volume is split in between two sections. A new section must be started, for instance, when the page-numbering style changes. This approach can not be used in such cases, because the toc-entry refers to a block and there is no volume break within a block.

[bertfrees]

Overall, I like this design because it is simpler than most of the other solutions we discussed. Other solutions tried to minimize the duplication, but I don't think that has been a goal of OBFL in the past, and it shouldn't be because the typical use case of OBFL is that it is generated from another document.

The only reservations I have are the following:

Firstly, I'm not sure yet about the names "toc-text" and "toc-text-resumed", but that's not the most important part and we still have some time to think about it.

Secondly, while letting the referenced blocks determine the "scope" of the toc-entrys is a logical thing to do, is simple and yet powerful, it is not a super good fit for CSS (which I am of course primarily interested in) and will moreover complicate my CSS-to-OBFL implementation a bit. The reason I say it's not a super good fit for CSS is because CSS basically considers the document structure as given. If the desired OBFL structure can not be accomplished purely with CSS, a pre-processing step is required. I haven't found an elegant solution for this problem. Maybe it's just something we need to live with. There are other parts of OBFL that are not very well suited for CSS, like the way the volume breaking is controlled (the keep-volume-priority properties). The fact that this will complicate my CSS-to-OBFL implementation is not really a reason to dismiss a design. It's just a remark.

Finally a suggestion to fix an ambiguity in the specification:

Every toc-entry refers to a block through attribute ref-id. If Dotify decides to apply a volume break such that the block is split accross several volumes, then toc-text-resumed is applied in all those volumes except the first one.

In OBFL we should state whether in case multiple nested blocks that are referenced by toc-entrys are split across volumes, all the corresponding toc-text-resumed are inserted in the new volume, or only the toc-text-resumed corresponding to the innermost block. For the strength of the design I think it would be nice if one could control this. There are various ways to achieve this, but letting it be determined by the document structure sounds the most logical. In other words, if you don't want multiple toc-text-resumed you need to make sure the referenced blocks are not nested.

[PaulRambags]

I would insert at most one toc-text-resumed.

It is the structure of the toc-entrys that determines which toc-text-resumed is inserted, not the structure of the corresponding blocks. So there is no need of a restriction on the block structure.
The innermost non-empty toc-text-resumed is inserted, if it exists.

Every toc-entry refers to a block. The information that I need is:

• the volume number of the corresponding block. This information is already available in the ContentReferenceHandler;
• whether that block is at the beginning (at the first body page) of the volume. This information can be added to the ContentReferenceHandler.