eg_rand_spec.rb

describe "Something" do

  before( :each ) do
    Kernel.stub!( :rand ).and_return( 0 )
  end

  def do_call
    MyObject.do_something
  end

  it "should generate a random number between 0 and 1" do
    Kernel.should_receive( :rand ).with( 2 ).and_return( 0 )
    do_call
  end
  
  context "when the random number is 0" do
  
    it "should be specced"
  
  end
  
  context "when the random number is 1" do
  
    before( :each ) do
      Kernel.stub!( :rand ).and_return( 0 )
    end
  
    it "should be specced"
  
  end

end