Pattern<TSource1>
Represents a join pattern over one observable sequence.
            
                namespace System.Reactive.Joins
{
    public class Pattern<TSource1> : Pattern
    {
        internal IObservable<TSource1> First { get; }
        internal Pattern(IObservable<TSource1> first)
        {
            First = first;
        }
        public Plan<TResult> Then<TResult>(Func<TSource1, TResult> selector)
        {
            if (selector == null)
                throw new ArgumentNullException("selector");
            return new Plan<TSource1, TResult>(this, selector);
        }
    }
}