Add RuleSet#could_be_empty?

lib/propane/parser/item.rb

@@ -72,6 +72,15 @@ class Propane
         end
       end
 
+      # Return whether the item is "complete", meaning that the parse position
+      # marker is at the end of the rule.
+      #
+      # @return [Boolean]
+      #   Whether the item is "complete".
+      def complete?
+        @position == @rule.components.size
+      end
+
       # Get the following symbol for the Item.
       #
       # That is, the symbol which follows the parse position marker in the

lib/propane/parser/item_set.rb

@@ -17,8 +17,8 @@ class Propane
       #   Maps a following symbol to its ItemSet.
       attr_reader :following_item_set
 
-      # @return [Set]
+      # @return [Set<ItemSet>]
-      #   Item sets leading to this item set.
+      #   ItemSets leading to this item set.
       attr_reader :in_sets
 
       # Build an ItemSet.
@@ -81,6 +81,18 @@ class Propane
         self == other
       end
 
+      # Set of ItemSets that lead to this ItemSet.
+      #
+      # This set includes this ItemSet.
+      #
+      # @return [Set<ItemSet>]
+      #   Set of all ItemSets that lead up to this ItemSet.
+      def leading_item_sets
+        @in_sets.reduce(Set[self]) do |result, item_set|
+          result + item_set.leading_item_sets
+        end
+      end
+
       # Represent the ItemSet as a String.
       #
       # @return [String]

lib/propane/rule.rb

@@ -42,6 +42,16 @@ class Propane
       @line_number = line_number
     end
 
+    # Return whether the Rule is empty.
+    #
+    # A Rule is empty if it has no components.
+    #
+    # @return [Boolean]
+    #   Whether the Rule is empty.
+    def empty?
+      @components.empty?
+    end
+
   end
 
 end

lib/propane/rule_set.rb

@@ -2,17 +2,41 @@ class Propane
 
   class RuleSet
 
+    # @return [String]
+    #   Name of the RuleSet.
     attr_reader :name
 
+    # @return [Array<Rule>]
+    #   Rules in the RuleSet.
     attr_reader :rules
 
+    # Construct a RuleSet.
+    #
+    # @param name [String]
+    #   Name of the RuleSet.
     def initialize(name)
       @name = name
       @rules = []
+      @could_be_empty = false
     end
 
+    # Add a Rule to the RuleSet.
+    #
+    # @param rule [Rule]
+    #   Rule to add.
     def <<(rule)
       @rules << rule
+      if rule.empty?
+        @could_be_empty = true
+      end
+    end
+
+    # Return whether any Rule in the RuleSet is empty.
+    #
+    # @return [Boolean]
+    #   Whether any rule in the RuleSet is empty.
+    def could_be_empty?
+      @could_be_empty
     end
 
   end