I am fitting a non-linear model within which I would like to use a moving average of an observed variable to calculate a non-linear parameter so that the parameter reflects changes observed over time in the observed variable.

@paul.buerkner @martinmodrak Is this possible with `brms`

nonlinear syntax?

An example:

```
# Some data
Data <- expand.grid(r = factor(c(1:5)),
t = factor(c(1:8))
) %>%
mutate(C = round(rnorm(nrow(.),500, 150)),
D = round(rnorm(nrow(.),10000, 1000)),
R = C/D,
S = rnorm(nrow(.),0.5, 0.15))
```

```
nl_priors <- c(
set_prior("normal(0, 10)",
class = "b",
nlpar = "a"),
set_prior("normal(0, 10)",
class = "b",
nlpar = "b"),
set_prior("normal(0, 10)",
class = "b",
nlpar = "c")
)
bform <- bf(
S ~ (C * B)/D,
nlf(B ~ a * R^b + c),
B ~ ma(gr = r),
a ~ 1,
b ~ 1,
c ~ 1,
nl = TRUE
)
```

```
NL_Mod <- brm(bform,
Data,
family = Beta(),
prior = nl_priors,
inits = "random",
iter = 2000,
warmup = 1000,
chains = 1,
cores = ncores,
backend = "cmdstan",
normalize = FALSE,
save_pars = save_pars(all = TRUE),
control = list(adapt_delta = 0.9,
max_treedepth = 12)
)
```

Which results in an error:

`Error: Explicit covariance terms can only be specified on 'mu'.`