diff --git a/lib/propane/grammar.rb b/lib/propane/grammar.rb
index 113f7c8..f14f230 100644
--- a/lib/propane/grammar.rb
+++ b/lib/propane/grammar.rb
@@ -44,7 +44,7 @@ class Propane
         elsif sliced = input.slice!(/\A(\S+)\s*->\s*(.*?)(?:;|<<\n(.*?)^>>\n)/m)
           rule_name, components, code = $1, $2, $3
           components = components.strip.split(/\s+/)
-          @rules << Rule.new(rule_name, components, code, line_number)
+          @rules << Rule.new(rule_name, components, code, line_number, @rules.size)
         else
           if input.size > 25
             input = input.slice(0..20) + "..."
diff --git a/lib/propane/rule.rb b/lib/propane/rule.rb
index a06147e..7778a12 100644
--- a/lib/propane/rule.rb
+++ b/lib/propane/rule.rb
@@ -10,6 +10,10 @@ class Propane
     #   User code associated with the rule.
     attr_reader :code
 
+    # @return [Integer]
+    #   Rule ID.
+    attr_reader :id
+
     # @return [Integer]
     #   Line number where the rule was defined in the input grammar.
     attr_reader :line_number
@@ -28,10 +32,13 @@ class Propane
     #   User code associated with the rule.
     # @param line_number [Integer]
     #   Line number where the rule was defined in the input grammar.
-    def initialize(name, components, code, line_number)
+    # @param id [Integer]
+    #   Rule ID.
+    def initialize(name, components, code, line_number, id)
       @name = name
       @components = components
       @code = code
+      @id = id
       @line_number = line_number
     end