```ruby
require 'spec_helper'

describe Mactag::Config do
  context 'configuration options' do
    it "should have option 'binary'" do
      Mactag::Config.should respond_to(:binary)
      Mactag::Config.should respond_to(:binary=)
    end

    it "should have option 'tags_file'" do
      Mactag::Config.should respond_to(:tags_file)
      Mactag::Config.should respond_to(:tags_file=)
    end

    it "should have option 'rvm'" do
      Mactag::Config.should respond_to(:rvm)
      Mactag::Config.should respond_to(:rvm=)
    end

    it "should have option 'gem_home'" do
      Mactag::Config.should respond_to(:gem_home)
      Mactag::Config.should respond_to(:gem_home=)
    end
  end

  it 'should be configurable' do
    Mactag.configure do |config|
      config.should == Mactag::Config
    end
  end

  it "requires '{INPUT}' and '{OUTPUT}' in binary string" do
    proc {
      Mactag.configure do |config|
        config.binary = 'ctags -o {foo} -e {bar}'
      end
    }.should raise_exception
  end
end
```

Make sure config can be changed.
user
```ruby
require 'spec_helper'

describe Mactag::Config do
  context 'configuration options' do
    it "should have option 'binary'" do
      Mactag::Config.should respond_to(:binary)
      Mactag::Config.should respond_to(:binary=)
    end

    it "should have option 'tags_file'" do
      Mactag::Config.should respond_to(:tags_file)
      Mactag::Config.should respond_to(:tags_file=)
    end

    it "should have option 'rvm'" do
      Mactag::Config.should respond_to(:rvm)
      Mactag::Config.should respond_to(:rvm=)
    end

    it "should have option 'gem_home'" do
      Mactag::Config.should respond_to(:gem_home)
      Mactag::Config.should respond_to(:gem_home=)
    end
  end

  it 'should be configurable' do
    Mactag.configure do |config|
      config.should == Mactag::Config

      config.binary = 'binary {INPUT} {OUTPUT}'
      config.tags_file = 'tags_file'
      config.rvm = false
      config.gem_home = 'gem_home'
    end

    Mactag::Config.binary.should == 'binary {INPUT} {OUTPUT}'
    Mactag::Config.tags_file.should == 'tags_file'
    Mactag::Config.rvm.should == false
    Mactag::Config.gem_home.should == 'gem_home'
  end

  it "requires '{INPUT}' and '{OUTPUT}' in binary string" do
    proc {
      Mactag.configure do |config|
        config.binary = 'ctags -o {foo} -e {bar}'
      end
    }.should raise_exception
  end
end
```
assistant