/ə/ should be neither +tense nor -tense

[chronodm]

Currently, /ə/ (mid central unrounded vowel) collides with /ɛ/ (low-mid front unrounded vowel). I think there's a couple of things going on here:

1. there should probably be an explicit front feature, so that the /ɛ/, /ɜ/, /ʌ/ trio would be +front, -back, -front, -back, -front, +back. But the file doesn't currently have /ɜ/ anyway, and this seems like a bigger change.

2. as I read Riggle's chart, /ɘ/, /ə/, and /ɜ/ are distinguished in that /ɘ/ is +tense, /ɜ/ is -tense, and
   /ə/ is neither:

   

   (The file doesn't currently have /ɘ/ either, but again, bigger change.)

I propose making setting /ə/ to "tense": 0, which will distinguish it from /ɛ/ and perhaps make it easier to add more vowels later.