m: dojoinstem

return:
a list of pluralised words: si_list (called si because this is what you need to
look for to make the singular)

the pluralised words as a dict of sets sorted by word length: si_bysize
the singular words as a dict of sets sorted by word length: pl_bysize
if dojoinstem is True: a regular expression that matches any of the stems: stem
N)