Manually scaling math fences

[Aditya Mahajan <adityam@umich.edu>]

Hi Hans,

There are instances where the automatic scaling provided by \left and 
\right is not ideal and one has to use manual scaling. For example:

\starttext
\startTEXpage \startformula
   \left\langle \sum_{i=1}^n A_i x_i \right\rangle
   \hbox{ vs. }
   \bigg\langle \sum_{i=1}^n A_i x_i \bigg\rangle
\stopformula \stopTEXpage
\stoptext

Currently, math fences always default to using \left and \right, which can 
lead to undesirable results. For example:

\definemathfence [MEAN] [angle] [command=yes, color=blue]

\starttext
\startTEXpage \startformula
   \MEAN{ \sum_{i = 1}^n A_i x_i }
   \hbox{ vs. }
   \bigg\langle \sum_{i=1}^n A_i x_i \bigg\rangle
\stopformula \stopTEXpage
\stoptext

Could we add an option to \definefence that allows the user to select the 
delimiter scaling using a key (say, `delimiter`) so that

\MEAN[delimiter=auto] expands to \left\langle .. \right\langle
\MEAN[delimiter=]     expands to \langle .. \rangle
\MEAN[delimiter=big]  expands to \big\langle  ... \big\rangle
\MEAN[delimiter=Big]  expands to \Big\langle  ... \Big\rangle
\MEAN[delimiter=bigg] expands to \bigg\langle ... \bigg\rangle
\MEAN[delimiter=Bigg] expands to \Bigg\langle ... \Bigg\rangle

I am not sure whether `delimiter` is the best key. I think that `scale` 
may be better, but using `scale` for specifying the scaling of math 
delimiters will be inconsistent with how `scale` is used everywhere else. 
Another option is to use `alternative` key.

Thanks,
Aditya
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