Hi,

I’m unable to find the special function lambert_w0 in my pystan/rstan program.

See doc and stan math pr.

I’m assuming here that stan-math functions should just eventually appear for use in rstan/pystan; though this might not be well-founded.

```
functions
{
int signum(real y)
{
return y > 0 ? 1 : -1;
}
}
data
{
int N;
real y[N];
}
parameters
{
real<lower=0> d;
}
transformed parameters
{
real z;
z = sign(y) * lambert_w0(d * y^2) / d; // doesn't work as lambert_w0 not found
}
model
{
d ~ gamma(1, 2);
z ~ normal(0, 1);
// todo jacobian adjustment since i'm putting a prior on a transformed parameter
}
```

I get a compile error and also

```
> rstan::lookup('rep')
StanFunction Arguments ReturnType
579 rep_row_vector (real x, int n) row_vector
580 rep_vector (real x, int m) vector
> rstan::lookup('lambert')
[1] "no matching Stan functions"
```