I know little about French, but I have a question about French phonology.
I saw that French has a rule of vowel nasalisation, which nasalises a vowel before an [n] in the same syllable. After vowel nasalisation, there is an n-deletion rule, which deletes [n] if it appears in the coda after a nasalised vowel. And finally, there is a schwa-deletion rule, which deletes foot-final [ə], so that the feminine forms do not have a schwa on the surface.
For example, the musculine "plein"[plɛ̃] and the feminine "pleine" [plɛn]. Their underlying forms are /plɛn/ and /plɛn-ə/.
However, it is said that there are also many words in French like "onde" [ɔ̃də], which only have a nasalised vowel, instead of an oral vowel following an [n]. Its underlying form is not /ɔndə/, like the "plein" does.
Then what is the underlying form of [ɔ̃də]?