just an example about how rspec works inside

rspe_in_test_unit

require 'test/unit'

class TestDescribe < Test::Unit::TestCase

  def test_that_it_can_pass
    describe "something" do
    
    end
  end
  
  def test_that_it_can_fail
    assert_raise(IndexError) do
      describe 'some failing thing' do
        it 'fails' do
          raise IndexError
        end
      end
    end
  end
  
  #Implementation
  def describe(description, &block)
    ExampleGroup.new(block).evaluate!
  end
  
  class ExampleGroup
    def initialize(block)
      @block = block
    end
    
    def evaluate!
      instance_eval(&@block)
    end
    
    def it(description, &block)
      block.call
    end
    
  end
  
end

class TestAssertion < Test::Unit::TestCase
  def test_that_it_can_pass
    2.should == 2
  end
end


class Object
  def should
    DelayedAssertion.new(self)
  end
end

class DelayedAssertion
  def initialize(subject)
    @subject = subject
  end
  
  def ==(other)
    self == other
  end
end