how do I test block given parameter

rspec_block_test.rb

require 'rspec'

class Foo
  def bar
    if block_given?
      yield(1)
      yield(2)
    end
  end
end

describe Foo do
  describe '#bar' do
    it 'should call by 1' do
      foo = Foo.new
      c = 0
      foo.bar do |a|
        a.should satisfy do |b|
          c += 1
          b == c
        end
      end
    end
  end
end