diff --git a/lib/imbecile/grammar.rb b/lib/imbecile/grammar.rb
index ca20e55..dcecf3f 100644
--- a/lib/imbecile/grammar.rb
+++ b/lib/imbecile/grammar.rb
@@ -1,18 +1,30 @@
 module Imbecile
   class Grammar
 
+    def initialize
+      @tokens = {}
+      @rules = {}
+    end
+
     # @return [Boolean]
     #   Whether loading was successful.
     def load(input_file)
       File.read(input_file).each_line.each_with_index do |line, line_index|
         line = line.chomp
         line_number = line_index + 1
-        if line =~ /^\s*token\s+(\S+)\s+(.*)$/
+        if line =~ /^\s*#/
+          # Skip comment line.
+        elsif line =~ /^\s*token\s+(\S+)\s+(.*)$/
           name, expr = $1, $2
           unless name =~ /^[a-zA-Z_][a-zA-Z_0-9]*$/
             $stderr.puts "Invalid token name #{name} on line #{line_number}"
             return false
           end
+          if @tokens[name]
+            $stderr.puts "Duplicate token name #{name} on line #{line_number}"
+            return false
+          end
+          @tokens[name] = expr
         end
       end
     end