# Re: [mathjax-dev] New menclose notation coming in MathML 3, 2nd edition

From: Frédéric WANG <fred.wang@free.fr>
Date: Mon, 09 Sep 2013 09:27:51 +0200
Message-ID: <522D7877.9040600@free.fr>

I am the one who proposed this notation, so perhaps I should say a word
on this.

First, as I said I've never seen this notation in France so I can't
comment about its usefulness other than believing you that it is (or
was) used at least in K12 math in the US. It's probably not fundamental
to have this notation (even the doc of the cancelto says that striking
formulas is a poor notation), but at least it was implemented
independently in two different projects (LaTeXML and MathJax) with
essentially the same method: menclose updiagonalstrike + a superscript
element. So it was important enough to need an implementation and the
MathML code to choose seems somewhat natural since the two developers
had the same idea. I don't like that menclose notation names don't take
the overall direction into account but here that seemed irrelevant if,
as I believe, it's not used in RTL math (the W3C Arabic Math notes
doesn't mention it). So I thought it was OK to add one menclose notation
for this particular arrow, but I definitely don't see the need for other
striking arrows if there is no use case.

The reason why I proposed this notation is that MathJax used to have a
non-standard class on the menclose element:

<menclose notation="updiagonalstrike" class="MJX-arrow">...</menclose>

to tell that the updiagonalstrike should have an arrow head. Hence
MathJax rendered the notation visually the same as the cancelto command
but other rendering engines like MathPlayer or Gecko just renders an
updiagonalstrike. Moreover, other TeX to MathML converter like LaTeXML
are not aware of MathJax's non-standard CSS classes and can not achieve
the same rendering via standard MathML (for example Bruce only used an
updiagonalstrike). Obviously, I don't like these specific hacks in
MathJax and I hope these nonstandard MJX-* classes will be implemented
via standard means in the future. In the case of cancelto, the menclose
notation is open-ended so I thought it would be easy to add a new
notation for that. Now MathJax and LaTeXML uses

<menclose notation="updiagonalstrike updiagonalarrow">...</menclose>

and this will be supported by Gecko 24 (to be released next week). I'd
like this to be in MathPlayer too, but even if that's not the case,
MathPlayer will just render the notation "updiagonalstrike" as it has
always done, which is understandable...

Le 08/09/2013 20:24, Neil Soiffer a écrit :
> The fact that it was one of the most requested features for MathJax is
> a strong argument for adding it to MathML.  I still remain dubious
> about its use though.  My kids have gone through/are going through K12
> math and I've never seen arrows used as part of crossouts in any of
> the books or online materials they used/we've looked at, either for
> cancellations or for carries/borrows. But that's just anecdotal
> evidence which is why I was hoping that Bruce Miller or Michael
> Kohlhase could give some input as to usage based on their arXiv work
> since they will see the macro and will have needed to implement it (or
> not) for LaTeX2XML.
>
> You also touched on a hot button topic for me:  I'm strongly against
> the idea that hacking solutions with mpadded or playing games with
> tables to achieve a particular presentation is acceptable. It means
> the result is not accessible and a certain percentage of the
> population is excluded from using the material. I'm not arguing for
> semantic usages, I'm just arguing that the presentation be able to be
> audibly described to the user in a meaningful manner.  E.g,. 'x minus
> 1' with a horizontal strike across it" is understandable and but not
> semantic.  However, playing games with mover and mpadded: "x minus 1
> with a line above it that has a negative depth of 15 pixels" is
> gibberish and if the mpadded is not spoken, (... with a line above
> it), the result is very misleading.
>
>     Neil
>
>
> On Saturday, September 7, 2013 8:46:36 AM UTC-7, dpvc wrote:
>
>     > The original use for the notation pointed to the TeX package
>     that implemented "cancelto".  But this command has another
>     argument which the arrow points to.  That doesn't fit into the
>     menclose notation.
>     >
>     > As you can see, I'm rather skeptical that adding this is a good
>     idea.  Perhaps Bruce or Michael can look at arXiv files and tell
>     us how many times cancelto is used in those files.  Maybe it is
>     much more common than I think it is.
>
>     Before the cancel package was added to MathJax, it was one of the
>     most requested extensions that we had.  It is used frequently at
>     the K-12 level, but not so much in higher level mathematics, so I
>     would be surprised if arXiv showed much use of it.  You would need
>     to look at educational material rather than research papers.
>
>     While I understand your desire for symmetry in providing all
>     possible arrows, I don't see the mathematical need for anything
>     but the lower-left to upper-right arrow, as I am not aware of any
>     other usage.  There certainly may be, but they would be far less
>     prevalent than this.  And to propose a lot of additional notations
>     for which there are no ready use cases, and then use that to
>     suggest that the one notation that DOES have use cases should not
>     be implemented seems a bit backhanded to me.
>
>     As for those other notations, I would say that vertical and
>     horizontal arrow notations are not needed because there are
>     already means of obtaining those arrows (even stretched to the
>     proper sizes, and they can be placed above or below with mover or
>     munder, to the left and right via juxtaposition, and struck
>     through vertically or horizontally with mpadded), while there is
>     no mechanism for obtaining diagonal arrows at arbitrary sizes.  If
>     you want to say that you need such arrows in all directions, and
>     with one or two arrowheads because they would be useful in
>     commutative diagrams, then I could see that (though it would be
>     awkward to use menclose for that), and this would lead to 6 new
>     notations (or 4 if you just to the heads separately); but
>     suggesting that the need for a cancelation arrow necessitates 23
>     other arrows seems to me to be taking the desire for symmetry too
>     far.
>
>     Davide
>
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