gistfile1.txt

ruby-1.9.1-p378 > class A
ruby-1.9.1-p378 ?>  def foo
ruby-1.9.1-p378 ?>    yield
ruby-1.9.1-p378 ?>    end
ruby-1.9.1-p378 ?>  def zoo
ruby-1.9.1-p378 ?>    puts "zoo"
ruby-1.9.1-p378 ?>    end
ruby-1.9.1-p378 ?>  end
 => nil 
ruby-1.9.1-p378 > a = A.new
 => #<A:0x00000100b53630> 
ruby-1.9.1-p378 > a.foo { |o| o.zoo }
NoMethodError: undefined method `zoo' for nil:NilClass
        from (irb):14:in `block in irb_binding'
        from (irb):7:in `foo'
        from (irb):14
        from /Users/mhorbul/.rvm/rubies/ruby-1.9.1-p378/bin/irb:17:in `<main>'
ruby-1.9.1-p378 >