Fix items slicing

Rakefile

@@ -1,4 +1,5 @@
 require "bundler/gem_tasks"
+require 'rake/testtask'
 
 task :examples do
   $LOAD_PATH.unshift File.expand_path('../examples', __FILE__)
@@ -7,3 +8,7 @@ task :examples do
   require 'recursive'
 end
 
+Rake::TestTask.new do |t|
+  t.libs << "test"
+  t.test_files = FileList['test/*_test.rb']
+end

lib/parser_combinator.rb

@@ -53,18 +53,20 @@ def flatten
       Items.new(super())
     end
 
-    def slice(nth)
-      if nth.instance_of? Range then
-        Items.new(super(nth))
-      else
-        super(nth)
+    def slice(*args)
+      case args.size
+      when 1
+        nth, = args
+        if nth.instance_of? Range then
+          Items.new(super(nth))
+        else
+          super(nth)
+        end
+      when 2
+        Items.new(super(*args))
       end
     end
 
-    def slice(pos, len)
-      Items.new(super(pos, len))
-    end
-
     def take(n)
        Items.new(super(n))
     end

test/items_test.rb

@@ -0,0 +1,8 @@
+require 'test_helper'
+
+class ItemsTest < Test::Unit::TestCase
+  def test_slice
+    items = ParserCombinator::Items.new([:a, :b, :c])
+    assert_instance_of ParserCombinator::Items, items.slice(1..2)
+  end
+end

test/test_helper.rb

@@ -0,0 +1,4 @@
+require "test-unit"
+
+$LOAD_PATH << File.join(__dir__, "../lib")
+require "parser_combinator"