[...] (X stands for compact grave consonants, S, for noncompact acute consonants, F, for noncompact grave consonants other than noninitial {*"m"}) [...]. - Halle (2004), a pag.25
Except after noninitial {*"m"} there can be n sequences of grave noncompact (labial) consonants. - Halle (2004), a pag.25