Keep track of item set in-links

lib/imbecile/parser.rb

@@ -34,6 +34,10 @@ class Imbecile
       @item_sets.each do |item_set|
         process_item_set(item_set)
         puts "Item set #{item_set.id}:"
+        ids = item_set.in_sets.map(&:id)
+        if ids.size > 0
+          puts "    (in from #{ids.join(", ")})"
+        end
         puts item_set
         item_set.follow_item_set.each do |follow_symbol, follow_item_set|
           if follow_symbol.is_a?(Token)
@@ -54,6 +58,7 @@ class Imbecile
         unless follow_symbol == @token_eof
           follow_set = @item_sets_set[item_set.build_follow_set(follow_symbol)]
           item_set.follow_item_set[follow_symbol] = follow_set
+          follow_set.in_sets << item_set
         end
       end
     end

lib/imbecile/parser/item_set.rb

@@ -11,9 +11,14 @@ class Imbecile
       #   Maps a follow symbol to its item set.
       attr_reader :follow_item_set
 
+      # @return [Set]
+      #   Item sets leading to this item set.
+      attr_reader :in_sets
+
       def initialize(items)
         @items = Set.new(items)
         @follow_item_set = {}
+        @in_sets = Set.new
         close!
       end