see also #14, #15, and #16, #17
As of 1/14/19, these are all the tags potentially in use:
Semantic markup ab cont div figure head id image lb/ man add><man margin page rub
Phrase-level tags al bp cn df env fr gk it la m md ms mu oc pa pl pn po pro sn tl tmp
Editorial markup add corr del exp gap sup unc