Dear All,

I am pretty new with brms and I was wondering if it is possible to specify a user defined function for parameters estimate of a non linear model.

In my case the dependent variables are a summation of other variables. A reproducible example could be something below:

```
### create data
set.seed(123)
class <-c(rep(3,10),rep(5,10),rep(6,10),rep(2,10),rep(8,10),rep(1,10),rep(7,10))
x=runif(length(class), 1,10)
### create the true response
allclasses=(unique(class))
dependent=rep(NA,length(allclasses))
for(k in 1:length(allclasses)){
deptemp=x[which(class==allclasses[k])]
sumvalt=sum(deptemp,na.rm=TRUE)
dependent[k]=sumvalt
}
response=2+5*dependent
### define the model
model<-function(vect1,class,param){
allclasses=(unique(class))
dependent=rep(NA,length(allclasses))
for(k in 1:length(allclasses)){
deptemp=vect1[which(class==allclasses[k])]
sumvalt=sum(deptemp,na.rm=TRUE)
dependent[k]=sumvalt
}
output=param[1]+param[2]*dependent
return(output)
}
```