finite_set.rb

require 'minitest/autorun'
require 'set'

class FiniteSet < Set
  def fetch(item)
    if member?(item)
      delete(item)
      item
    end
  end
end

class FiniteSetTest < MiniTest::Unit::TestCase
  def setup
    @set = FiniteSet.new (1..10).to_a
  end

  def test_fetch_returns_item_requested_if_in_set
    assert_equal 5, @set.fetch(5)
  end

  def test_fetch_returns_nil_if_item_not_in_set
    assert_equal nil, @set.fetch(50)
  end

  def test_fetch_removes_item_requested_from_set
    (1..10).to_a.each {|n| @set.fetch(n) if n.even?}
    assert_equal FiniteSet.new([1, 3, 5, 7, 9]), @set
  end
end

Nice! Totally personal preference now, but for code like this, I also like to use guard clause:

class FiniteSet < Set
  def fetch(item)
    return unless member?(item)

    delete(item)
    item
  end
end