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Patrick:
This is an alternative suggestion for an analysis of the *o-stem genitive
plural in *-o:m.
I propose that the last earliest neuter nominative singular in -*om formed
a nominative plural by adding -*Ha(:) to -*om, producing -*om&, and
subsequently -*o:m.
The late neuter genitive plural was differentiated from it by shift of
stress-accentuation, forming an adjective in -*ó:m.
Subsequently, the neuter nominative plural was re-formed with -*Ha(:),
leaving -*ó:m as the neuter genitive plural _without_ a prosodic counterpart in
the nominative.
This neuter genitive plural form was subsequently adopted by the
-*o-stems.
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