features: combining plus sign below

[drammock]

This diacritic currently has no features associated with it, so phonemes with and without the diacritic that are otherwise identical will be featurally indistinct.

[drammock]

a further problem is that I thought that the combing plus sign below had been restricted to consonants only in this project since very early on. However, there seem to be quite a lot of cases of a̟ in the current data... running:

all.data[grepl("a̟", all.data$Phoneme, fixed=TRUE),]

returns 66 lines, for example.