How to use conditionals?

Imagine a model with two binary data vectors `a` and `b`. Both represent events that can either occur or not occur. However, `b` is impossible when `a` does not occur. I'd like to model the probability that `b` occurs given that `a` occurs with a probit model.

In stan, I can do this with a conditional statement, e.g.

```
p[i] = (a[i] == 0) ? 0 : Phi( b0 );
```

which just makes the probability that `b` occurs equal to 0 whenever `a` does not occur; when `a` does occur, I get my probit model. This seems to work well.

In greta, I don't see that there are conditions. I'm trying to emulate this with a multiplication:

```r
library(greta)

ar <- rep(c(0,1), each = 100)
br <- rep(c(0,1), 100)
br[a0 == 0] = 0
table(ar,br)

a <- as_data(ar)
b <- as_data(br)

b0 <- normal(0,1)
p <- (a == 0) * iprobit( b0 )
distribution(b) <- bernoulli( p )

m <- model(b0)
plot(m)

inits <- initials( b0 = 0)

draws <- mcmc(m, n_samples = 1000, initial_values = inits)
```

So whenever `a` is 0, the probability is 0; else, it is Phi(b0). However, I cannot get this model to run. I get

```
Error: The log density could not be evaluated at these initial values.
Try using calculate() see whether they lead to values of other greta arrays in the model.
```

When I remove the `(a == 0) * ` bit, the model runs fine, but gives me the marginal estimate of the probability (which is not what I want, of course).

What's the best way of accomplishing this in greta?