Systematically locate elements allowed only once in a context, and check for bad examples showing multiple usage

[ebeshero]

Referencing https://github.com/TEIC/TEI/issues/1526, Lou just corrected a couple of egXML examples in the Guidelines in which we were showing multiple uses of an element in a context where it is really only permitted once. Per @sydb 's recommendation, we should look systematically for more of these.

Syd recommends that we "write software that a) reads in the Guidelines (or perhaps, the schema generated therefrom), and generate a list of elements that may occur a maximum of once in a particular context, and b) searches for any case of two or more of any of those elements within an <egXML> that either are in that context or have no context."

[jamescummings]

I would do so on the full Guidelines because the generated schema (and p5subset) will be missing egXMLs from the prose.

[martindholmes]

@ebeshero and I believe this is actually a TEI issue, not a Stylesheets issue, unless we believe that this sort of diagnostic should be a formal part of the Stylesheets.

[ebeshero]

And we note that this is connected with https://github.com/TEIC/TEI/issues/1991