Add function to create combination model, given a mable

Having a function automatically create a simple average combination model would be very convenient. Here's a simple way it could work -

library(tsibble)
library(fable)

addCombn <- function(m, name) {
  mv <- mable_vars(m)
  m[[name]] <- Reduce(f = '+', as.list(m[, mv])) / length(mv)
  return(m)
}

m <- tourism %>%
  dplyr::filter(Region == 'Adelaide' & Purpose == 'Business') %>%
  model(
    E = ETS(Trips), 
    A = ARIMA(Trips), 
    T = THETA(Trips)
  ) %>%
  addCombn('EAT')

This almost works as desired. See below -

mable_vars(m) # misses 'EAT'
[1] "E" "A" "T"
m$cc <- 1L
m$cc <- NULL
mable_vars(m) # desired outcome
[1] "E" "A" "T" "EAT"

I think there's something subtle that I'm missing, maybe some shallow copies?

Start with the example in the issue and inspect how mable_vars() identifies model columns and how model() constructs a mable. Determine how a newly added combination model can be registered so mable_vars(m) includes it; done means the proposed helper creates the simple average model and the resulting mable reports the new name.