Class based contextual positioning - how to?

I've tried using calt but it's not really working - i.e. it works fine in FontForge but generated fonts seem to ignore it. 

feature calt {
  pos @wide @narrow' <128 0 0 0> @narrow;
} calt; 

You can see the feature file here: https://github.com/larsenwork/monoid/blob/master/Utilities/RegularKerning.fea

I can't use standard kerning as the font is monospaced. 

Any suggestions? (I'm a bit new to all this - just playing around with the possibilities.