Creates a nested example group named by the submitted `attribute`, and then generates an example using the submitted block.

@example

  # This ...
  describe Array do
    its(:size) { should eq(0) }
  end

  # ... generates the same runtime structure as this:
  describe Array do
    describe "size" do
      it "should eq(0)" do
        subject.size.should eq(0)
      end
    end
  end

The attribute can be a `Symbol` or a `String`. Given a `String` with dots, the result is as though you concatenated that `String` onto the subject in an expression.

@example

  describe Person do
    subject do
      Person.new.tap do |person|
        person.phone_numbers << "555-1212"
      end
    end

    its("phone_numbers.first") { should eq("555-1212") }
  end

When the subject is a `Hash`, you can refer to the Hash keys by specifying a `Symbol` or `String` in an array.

@example

  describe "a configuration Hash" do
    subject do
      { :max_users => 3,
        'admin' => :all_permissions.
        'john_doe' => {:permissions => [:read, :write]}}
    end

    its([:max_users]) { should eq(3) }
    its(['admin']) { should eq(:all_permissions) }
    its(['john_doe', :permissions]) { should eq([:read, :write]) }

    # You can still access to its regular methods this way:
    its(:keys) { should include(:max_users) }
    its(:count) { should eq(2) }
  end

With an implicit subject, `is_expected` can be used as an alternative to `should` (e.g. for one-liner use)

@example

  describe Array do
    its(:size) { is_expected.to eq(0) }
  end

You can pass more than one arguments on the `its` block to add some options to the generated example

@example

  # This ...
  describe Array do
    its(:size, :focus) { should eq(0) }
  end

  # ... generates the same runtime structure as this:
  describe Array do
    describe "size" do
      it "should eq(0)", :focus do
        subject.size.should eq(0)
      end
    end
  end

Note that this method does not modify `subject` in any way, so if you refer to `subject` in `let` or `before` blocks, you‘re still referring to the outer subject.

@example

  describe Person do
    subject { Person.new }
    before { subject.age = 25 }
    its(:age) { should eq(25) }
  end