I am following this tutorial, but the examples do not run.

I have compiled successfully models with pystan recently, but these examples without model or data block seem problematic.

The first example is:

```
data {
int<lower=0> N;
real<lower=0, upper=1> theta;
}
generated quantities {
int<lower=0, upper=N> y = binomial_rng(N, theta);
}
```

The python code is:

```
import stan
N = 100
θ = 0.3
with open('model.stan', 'r') as f:
model = f.read()
data = {'N': N, 'theta': θ}
posterior = stan.build(model, data, random_seed=123)
res = posterior.sample(num_chains=1, num_samples=10, num_warmup=0)
df = res.to_frame()
print(df.head())
```

The program stops at the line `res = posterior.sample`

, and I get an AssertionError. Stan warns me that *Posterior is improper. Please check your model.*

Similarly, if I try the stan code:

```
generated quantities {
real<lower=-1, upper=1> x = uniform_rng(-1, 1);
real<lower=-1, upper=1> y = uniform_rng(-1, 1);
int<lower=0, upper=1> inside = x^2 + y^2 < 1;
real<lower=0, upper=4> pi = 4 * inside;
}
```

I have the same problems.

- Ubuntu 20.04
- Python 3.9
- PyStan 3.7