I am following the *Applied Gaussian Process in Stan* document by @drezap (Applied Gaussian Processes in Stan, Part 1. A Case Study).

```
transformed parameters {
matrix[N, N] L_K;
{
matrix[N, N] K;
K = gp_exp_quad_cov(X, magnitude, length_scale);
K = add_diag(K, square(eta));
L_K = choleky_decompose(K);
}
}
```

but when I try to comple the model I get the following error:

```
SYNTAX ERROR, MESSAGE(S) FROM PARSER:
No matches for:
gp_exp_quad_cov(vector[], real, real)
Function gp_exp_quad_cov not found.
```

Is still this function in the development code and must I implement the kernel as a Stan function?

The version of stan that I use is:

```
rstan_2.18.2 StanHeaders_2.18.0
```