A string begins with a brivla if it does not begin with a cmavo and consists of any number of initial-rafsi (possibly none) followed by a brivla-core.
A brivla-core is the part of a brivla that carries the penultimate stress. It can be a fu'ivla, a gismu, a CVV-final-rafsi or a short-lujvo.
A short-lijvo consists of a stressed-initial-rafsi and a short-final-rafsi.
Note that a CVV-final-rafsi needs at least one initial-rafsi in front in order to constitute a brivla, otherwise it will be taken as a cmavo.
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A lujvo is any brivla that is not only a gismu or only a fu'ivla.