Actually, to be consistent with PyMC2 we would probably call this "Lambda", but since we aren't using lambda functions, that would be more confusing.

Since the only reason we need to be doing this at all is to ensure that we can easily add the deterministic to the trace, is it wort calling it something like "add_var" (or similar) and have the function not only name it, but also add it to the `vars` list?  Or would this cause problems when evaluating the model likelihood? This way, we can simply have the trace track everything in `model.vars`. If its a problem, I wonder if it would be worth having a `model.nodes` attribute that is the union of the stochastics and deterministics (or a function that generates this union as needed, such as for the trace and for plotting).