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OpenType Layout Features - Class
The class keyword is used to declare a class of glyphs to be used for Chained Context Substitution and several GPOS lookup types.
class @<class name> [ <glyphs> ];
•name is a name you can create yourself. You may only use a-z, A-Z, 0-9 and "_" in your name and it must start with an @ (at) sign. Names are case-sensitive, and may only be declared once.