Re: Proposal for isolation characters in Unicode and the unicode-bidi:isolate and unicode-bidi:plaintext definitions

On Sun, Jul 8, 2012 at 6:52 AM, Aharon (Vladimir) Lanin
<aharon@google.com>wrote:

> 1. Make isolates weaker than embeddings/overrides, i.e. ignore a PDI when
> a PDF is expected, and have a PDF close all isolates opened between it and
> its marching LRE/RLE/LRO/RLO. Thus, in a, the PDI is ignored, and in b, the
> PDF ends the scope of the RLI as well as the LRE.
>
> 2. Vice-versa - make isolates stronger than embeddings/overrides, i.e.
> ignore a PDF when a PDI is expected, and have a PDI close all
> embeddings/overrides opened between it and its marching FSI/LRI/RLI.
> Thus, in a, the PDI ends the scope of the RLE as well as the LRI, and in b,
> the PDF is ignored.
>
> Possibility 2 offers greater forward compatibility, since new and old apps
> will interpret the PDFs as closing the same scopes when isolates are not
> properly nested with respect to embeddings/overrides.
>
> Possibility 1, on the other hand, gives isolates the desirable feature of
> isolating their surroundings from their contents - even when their contents
> contains extra or missing PDFs.
>
> I have decided to go with possibility 2, since IMO
> forward compatibility is not very important for what are, essentially,
> broken documents.
>

I don't understand your logic. You say option 2 offers greater forward
compatibility, but then say you are choosing 2 because forward
compatibility is NOT important.

I think backward compatibility is more desirable, i.e., a system that knows
nothing of isolates should work without modification, and yet option 2
requires PDI to close an embedding/override, which would violate that goal.
You should choose option 1 (PDI weaker than PDF).