Stan model for MacKay's Chapter 3 particle detector example.

particles.stan

data {
  int N;
  real<lower=1, upper=20> observations[N];
}
parameters {
  real<lower=0> lambda;
}
model {
  real Z = exp(-1/lambda) - exp(-20/lambda);
  for (i in 1:N) {
    target += exponential_lpdf(observations[i] | 1/lambda) - log(Z);
  }
}