Why have two levels? Because it should make writing
morphological rules easier down the road. Say you have an
inflectional prefix |e-| and you want to add it to oskal and get
eskal rather than *eoskal (or whatever) even though that's not
how it works with other vowel-initial words. In that case, you
just order your rules so epenthesis applies at the very end,
after all the prefixes and suffixes have been added. It's easier
to illustrate with examples. Let's take two German words,
['tʰaːkʰ] "day" and ['ʃpaːtʰ] "spar [a kind of mineral]". Both
of these take the plural ending |ə|, but although the plural of
[ʃpaːtʰ] is ['ʃpaːtʰə], the plural of ['tʰaːkʰ] isn't
*['tʰaːkʰə]. Instead, it's ['tʰaːgə]. How do you deal with an
ending that voices some stops but not others? Well, if you
notice that voiced stops (e.g. [g], [d], etc.) never occur in
final position in German, then you can write a final devoicing
rule . Your underlying forms can have voiced stops in this
position but then a phonological rule replaces them w