Dear Stan/brms community,

I am currently having trouble when mixing multilevel with nonlinear models.

I want to introduce harmonic terms (sine wave) in the model. The following works:

```
pois_form <- bf(y ~ a*sin(b*t) + sex, a+b~1+sex,nl=T)
ab_priors <- prior(cauchy(0, 1), nlpar = "a", lb = 0) +
prior(cauchy(0, 1), nlpar = "b",lb = 0)
poiss_fit <- brm(pois_form, prior = ab_priors,
data = gp_df, family = poisson())
```

However, when introducing a random effects term (either with **(1 |gr(ParticipantID))** or **(1 |ParticipantID))**), I get a parsing error which seems to be related to the multilevel term in the Stan output code:

My formula becomes:

```
pois_form <- bf(y ~ a*sin(b*t) + sex + (1 |gr(ParticipantID)), a+b~1+sex,nl=T)
```

Which returns the follwowing error

```
SYNTAX ERROR, MESSAGE(S) FROM PARSER:
error in 'model24a9763592c2_file24a96eaf1d9c' at line 36, column 59
-------------------------------------------------
34: for (n in 1:N) {
35: // compute non-linear predictor values
36: mu[n] = nlp_a[n] * sin(nlp_b[n] * C_1[n]) + C_2[n] + (1|gr(C_3[n]));
^
37: }
-------------------------------------------------
PARSER EXPECTED: ")"
Error in stanc(model_code = paste(program, collapse = "\n"), model_name = model_cppname, :
failed to parse Stan model 'file24a96eaf1d9c' due to the above error.
```

Am I missing something? Any ideas on how to fix it? I would also welcome any material on spectral decomposition (Fourier terms) for multilevel modeling considering covariates.