- Operating System: Ubuntu 16.04.4
- brms Version: 2.2.0

Hello!

I have trouble coding beta distributional modelling with brms.

`my_formula <- bf(y ~ x1 + x2, phi ~ x1 + x2, family = Beta)`

The operators for the reparametrization step are only adapted for vector * real, so the compilator return this error :

```
SYNTAX ERROR, MESSAGE(S) FROM PARSER:
No matches for:
vector * vector
Available argument signatures for operator*:
real * real
vector * real
row vector * real
matrix * real
row vector * vector
vector * row vector
matrix * vector
row vector * matrix
matrix * matrix
real * vector
real * row vector
real * matrix
expression is ill formed
error in 'model62bd888b6e_file62b1ff3659' at line 51, column 37
-------------------------------------------------
49: // likelihood including all constants
50: if (!prior_only) {
51: target += beta_lpdf(Y | mu * phi, (1 - mu) * phi);
^
52: }
-------------------------------------------------
Error in stanc(model_code = paste(program, collapse = "\n"), model_name = model_cppname, :
failed to parse Stan model 'file62b1ff3659' due to the above error.
```

The operator should be .*, or a loop is needed. I will be able to solve this by formulating my own beta distribution, but the issue deserves to be reported!

Thank you!

Lucas