public void OptParser()
{
var stream1 = new MemoryStream <char>("a".AsMemory());
var stream2 = new MemoryStream <char>("b".AsMemory());
var p = Combinator.Opt(Combinator.Char('a'));
var r1 = p.Parse(stream1);
var r2 = p.Parse(stream2);
Assert.True(r1.IsOk);
Assert.True(r2.IsOk);
Assert.Equal('a', r1.Ok.Value);
Assert.Equal(default, r2.Ok.Value);
