public void Proposal_AddAtPosition()
{
var proposal = new Proposal("セクション");
var section1 = new Section("セクション1");
var section2 = new Section("セクション2");
proposal.Add(section1, section2);
CollectionAssert.AreEqual(new[] { section1, section2 }, proposal.Sections.ToArray());
var section3 = new Section("セクション3");
proposal.AddAt(2, section3);
CollectionAssert.AreEqual(new[] { section1, section3, section2 }, proposal.Sections.ToArray());
}