THe replace operator in Omega allows us to ask what-if questions. This raises thorny problems that are both philosophcal and technical. In particula, when we construct a counter-factual what-if scenario, what should be preserved from the "real" world to the counterfactual world?

Consider the following example:

coin = bernoulli(0.5, Bool)
function x_(rng)
  coin_ = coin(rng)
  if coin
    @show b = normal(rng, 0, 1)
    @show b = normal(rng, 0, 1)
  (coin = coin_, b = b)
x =~ x_

Let's draw a sample where y is false

rand(x, )

In a counterfactual world, we replace force y to be true

xnew = replace(x, y => true)
label(x, nm) =~ w -> (println("in $nm"); x(w))
xsum = label(xnew, "xnew") + label(x, "x")