Dear Paul,

I have now been working with phylogenetically corrected models in brms for the past two years. I noticed that the previously used cov_ranef function was now replaced with a grouping argument inside the gr function. However, when I tried the implement the same structure as the one I used with cov_ranef, I get the following error: Grouping structure ‘gr’ expects only a single grouping term. I am not well versed enough to understand specifically what it means as I only have one grouping factor in my very simple database. I therefore looked at the example that was posted in the phylogenetic vignette of brms and copied it into my R session. However, I still get the same error when trying to run the model… Would you be able to help me on how to solve this issue?

All the best!

Antoine

Session info:

R version 4.0.0 (2020-04-24)

Platform: x86_64-w64-mingw32/x64 (64-bit)

Running under: Windows 10 x64 (build 18363)

```
phylo <- ape::read.nexus("https://paul-buerkner.github.io/data/phylo.nex")
data_simple <- read.table(
"https://paul-buerkner.github.io/data/data_simple.txt",
header = TRUE
)
A <- ape::vcv.phylo(phylo)
model_simple <- brm(
phen ~ cofactor + (1|gr(phylo, cov = A)),
data = data_simple,
family = gaussian(),
data2 = list(A = A),
prior = c(
prior(normal(0, 10), "b"),
prior(normal(0, 50), "Intercept"),
prior(student_t(3, 0, 20), "sd"),
prior(student_t(3, 0, 20), "sigma")
)
)
Output: Error: Grouping structure 'gr' expects only a single grouping term
```

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