Calculate follow token set for an ItemSet

lib/propane/parser/item_set.rb

@@ -25,6 +25,10 @@ class Propane
       #   Reduce actions, mapping lookahead tokens to rules.
       attr_accessor :reduce_actions
 
+      # @return [Set<Token>]
+      #   Follow set for the ItemSet.
+      attr_accessor :follow_set
+
       # Build an ItemSet.
       #
       # @param items [Array<Item>]