So my guess is it allows you to add basic inflectional morphology to the words on its list. So "write" is on the list, so you can use "writing", but "writing" itself isn't on the list, so you can't use "writings". And then, you know, "find" is on the list, so I could use "finded" if I wanted to, but of course "finded" isn't a word, so "found" has to be on the list too, so it has to allow "founded" too, so.... Well, anyway.