Provide some kind of "Retrieve" or "Sample" method, which returns one of the objects *already* built by a builder #37

This may be out of the scope of how you're using the library, but one of the most common cases that we had to end up working around at work is telling the builder to use an existing object: for example, you want to create a bunch of classes, and then distribute them among a bunch of professors or the like.

It's not enough to simply use builders for the property, as then there would be no overlap: every course would get a distinct instructor, and often subtle bugs in testing only arise when there is this kind of overlap.

The solution we've been using is to generate a list of instructors, and then for each course that gets built, pick a random element from this list. This is a relatively tiny bit of boilerplate, but it gets annoying when you're doing it in many different tests.

I want to add a "Sample" or "Retrieve" or "Past" method to a builder (naming things is hard, ok?) which can accomplish this with no boilerplate, and give code that looks like below:

var instructors = Builder<Instructor>.CreateListOfSize(10).All().WithTenure();
var courses = Builder<Course>.CreateListOfSize(100)
                             .All().Set(x => x.Instructor, () => instructors.Sample());

This is close to perfect, and lets you generate trees fairly easily (which we do all the time in our tests):

var nodes = Builder<OrgUnit>.CreateListOfSize(1000);
nodes.TheFirst().Set(x => x.Parent, null)
     .TheRest().Set(x => x.Parent, () => nodes.Sample());

However, it doesn't let you (easily) create random graphs. If we wanted that, we'd have to do something like: Create all of the object references, then start applying the property modifications to them. This would actually accomplish what we wanted to do elsewhere too, where your lambda could take in the object you were building in a partially constructed state, so that your choice of value could depend on the value chosen. (i.e. give the professor a salary over a specific amount if they had tenure). However, this also may be over-kill.

I have a crude prototype of this, but I'd like your thoughts on how much of the above is worthwhile and if you have any thoughts about the implementation. (i.e., anything more than just a private list that we sample from?)