A string begins with a fu'ivla if it does not begin with a cmavo or a rafsi-string or a slinku'i, it does begin with a non-h onset, and consists of any number of unstressed-syllables (possibly none), one stressed-syllable and then a final-syllable.
A rafsi-string consists of any number of y-less-rafsi (possibly none) followed by a gismu, a CVV-final-rafsi, or a stressed-y-less-rafsi and a short-final-rafsi
A slinku'i consists of a consonant followed by a rafsi-string.