# Re: mfenced as a special case of mrow -- Was: angle brackets in math

From: David Carlisle <davidc@nag.co.uk>
Date: Wed, 28 May 2008 23:58:31 +0100
Message-Id: <200805282258.m4SMwV5n015898@edinburgh.nag.co.uk>



> But any kid who starts typing MathML and enters
>
>     <mrow><mi>a</mi><mi>b</mi></mrow>
>
> and in his web browser sees
>
>               ab
>
> and then enters
>
>     <mfenced><mi>a</mi><mi>b</mi></mfenced>
>
> and sees
>              (a,b)
>
> and then finds the three attribute names "open", "close", and
> "separators" is likely to make the inference that it's a list
> constructor.
>

My own kid (who's 4 and a half so just about ready to type mathml) would
probably already recognise brackets in 1 + (2  + 3)  but I suspect it
will be a year or two or ten before concepts such as list really come
into his inferences.

> Then there's the name of that third attribute.  If things like "+",
> "*", and "&invisibletimes;" are suitable there, why isn't the name of
> third attribute "operators" and why isn't the default value of
> "separators" "&invisibletimes;" (which is the default separator in
> chalk-talk)?
No you shouldn't use + as the separator but I routinely mark up
1 + (2 + 3)
as

mrow
mn 1
mo +
mfenced separators=""
mn 2
mo +
mn 3

or equivalently

mrow
mn 1
mo +
mfenced
mrow
mn 2
mo +
mn 3

> But I still think it _might_ be possible to break slightly from the
> past and take advantage of mfenced's "open" and "close" for service in
> the role of LaTeX's \leftX...\rightY

mapping to \left  \right was probably the intuition behind adding mfenced
in the first place (although the mathml model of stretching to the size
of the parent mrow is strictly more powerful than the TeX/mfenced model
of stretching to the size of the enclosed expression.

> so as to finesse the confusion with balancing pairs.
?

David

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