Proto-Slavic *o~ and *U(n) [was: Vampire]

> From: Piotr Gasiorowski [mailto:piotr.gasiorowski@...]
> Subject: [tied] Re: Vampire [was: Pagan, heathen ...]
>
>The Slavic forms can
> be derived from *an, which developed two different Proto-Slavic
> variants: "strong" *o~ found in compounds (with word-medial treatment
> of the final nasal) and "weak" *U(n) used as a preposition or preverb
> meaning 'in' (with word-final phonetics, the final *n being realised
> only before a vowel, like a/an in English).
>

Since the word-medial reflex of *an is actually indistinguishable from
that of *un (< *n.) in Proto-Slavic, it's formally possible to derive
both Proto-Slavic *o~ and *U(n) from earlier *un, isn't it? (I'm not
quite sure about "necessitas" and "ratio" as to *an in that case). And
where do we find that *an in Baltic? Lithuanian seems to only have /in/
(~[i,]) (< *n., frontishly vocalized) and *-én (a locative-forming
postfix, if indeed from *h1en -- why acute? *-e-h1n?), Old Prussian --
/en/. From a minimalistic point of view, it would be nice to confine
ourselves to the e-grade only.

Sergei