[use] Allow multiple FMs in a cluster

[dscorbett]

Instead of overriding certain characters to F (#525, #1399), this PR simply allows multiple FMs in a cluster. FM is split into FMAbv, FMBlw, and FMPst. Only one FMPst is allowed, because all FMPsts are non-marks, so the USE spec never inserts a dotted circle before one: if another FM precedes an FMPst, the FMPst just becomes an independent cluster. I didn’t want to deviate from the spec in a case where the current rendering would not get a dotted circle.

[behdad]

Only one FMPst is allowed, because all FMPsts are non-marks, so the USE spec never inserts a dotted circle before one: if another FM precedes an FMPst, the FMPst just becomes an independent cluster. I didn’t want to deviate from the spec in a case where the current rendering would not get a dotted circle.

But then why include FMPst at all? I don't like the "|" in there. Feels unjustified.

[dscorbett]

Currently, using my new categorization but the old behavior, C FMPst FMPst is two clusters: C FMPst and FMPst. If the machine used FMPst*, there would be one cluster. If I got rid of FMPst, there would be three clusters. I prefer following the spec, even when it’s a little weird, as long as it isn’t breaking anything, i.e. inserting dotted circles in valid text.

Unlike other *Pst categories, FMPst contains no marks, which is why it behaves differently than FMAbv and FMBlw. I had considered calling the category FMEnd to make it more obviously different. Would that help?

[behdad]

Thanks.