In the series of OI personal numerals, why is "sesser" (six persons)
followed by "mórfesser" (great-six persons = seven persons) rather
than the logical "sechtar", based on "secht"? Modern Irish uses
"seachtar" normally, although "mórsheisear" is not unknown.
Was avoidance of confusion with the preposition "sechtar" (outside,
beyond) the motive? Or was there some sort of taboo avoidance or
other consideration at work here?