lookup underscore_arrows {
  # middle parts
  sub [underscore_start.uarw underscore_middle.uarw] bar' underscore by bar_middle.uarw;

  sub [underscore_start.uarw underscore_middle.uarw bar_middle.uarw] underscore' bar underscore by underscore_middle.uarw;
  sub [underscore_start.uarw underscore_middle.uarw bar_middle.uarw] underscore' underscore by underscore_middle.uarw;

  # ends
  sub [underscore_start.uarw underscore_middle.uarw bar_middle.uarw] underscore' by underscore_end.uarw;
  
  # beginnings
  sub underscore' bar underscore by underscore_start.uarw; 
  sub underscore' underscore by underscore_start.uarw; 
} underscore_arrows;