font-FiraCode/features/calt/underscores.fea

lookup underscores {
  # middle parts
  sub [underscore_start.seq underscore_middle.seq] bar' underscore by bar_underscore_middle.seq;

  sub [underscore_start.seq underscore_middle.seq bar_underscore_middle.seq] underscore' bar underscore by underscore_middle.seq;
  sub [underscore_start.seq underscore_middle.seq bar_underscore_middle.seq] underscore' underscore by underscore_middle.seq;

  # ends
  sub [underscore_start.seq underscore_middle.seq bar_underscore_middle.seq] underscore' by underscore_end.seq;
  
  # beginnings
  sub underscore' bar underscore by underscore_start.seq; 
  sub underscore' underscore by underscore_start.seq; 
} underscores;