public sealed class MySequence : NativeActivity
{
    Collection<Activity> children;
    Collection<Variable> variables;
    Variable<int> currentIndex;
    CompletionCallback onChildComplete;

    public MySequence()
        : base()
    {
        this.children = new Collection<Activity>();
        this.variables = new Collection<Variable>();
        this.currentIndex = new Variable<int>();
    }

    public Collection<Activity> Activities
    {
        get
        {
            return this.children;
        }
    }

    public Collection<Variable> Variables
    {
        get
        {
            return this.variables;
        }
    }