For each data frame, I want a unique identifier. I already can do this using a for loop, but I'm trying to make my code more R-like, and I feel like this could be done with an `apply` function or `dplyr`. My current code example:

```
df1 <- data.frame(x="ayyy", y=rnorm(5))
df2 <- data.frame(x="lmao", y=rnorm(6))
l1 <- list(df1,df2)

for (i in seq_along(l1)){
  l1[[i]] <- cbind(l1[[i]],i)
}

l1

```

I would also appreciate other methods of achieving this end.