Your question is answered in the Advanced Conextual Alternates tutorial, please read it to the end:
https://www.glyphsapp.com/tutorials/features-part-3-advanced-contextual-alternates
Your question is answered in the Advanced Conextual Alternates tutorial, please read it to the end:
https://www.glyphsapp.com/tutorials/features-part-3-advanced-contextual-alternates