Hello, I started the ml elixir book, the first code in this book has error for me

require Explorer.DataFrame, as: DF
iris = Explorer.Datasets.iris()
cols = ["sepal_width", "sepal_length", "petal_length", "petal_width"]

DF.mutate(iris,
  for col <- across(cols) do
    {col.name, (col - mean(col)) / variance(col)}
  end
)

Errors

** (ArgumentError) undefined variable "cols"
    (explorer 0.6.1) lib/explorer/query.ex:365: Explorer.Query.traverse/3
    (elixir 1.15.2) lib/enum.ex:1819: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
    (explorer 0.6.1) lib/explorer/query.ex:375: Explorer.Query.traverse/3
    (explorer 0.6.1) lib/explorer/query.ex:401: Explorer.Query.traverse_for/3
    (explorer 0.6.1) lib/explorer/query.ex:298: anonymous fn/3 in Explorer.Query.traverse/2
    (elixir 1.15.2) lib/enum.ex:1819: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
    (explorer 0.6.1) lib/explorer/query.ex:296: Explorer.Query.traverse/2
    (explorer 0.6.1) expanding macro: Explorer.Query.query/1

I am using LiveBook v0.10.0 and these deps:

Mix.install([
  {:axon, "~> 0.5"},
  {:nx, "~> 0.5"},
  {:explorer, "~> 0.5"},
  {:kino, "~> 0.5"}
])

It should be noted if I use the list directly it has not problem

require Explorer.DataFrame, as: DF
iris = Explorer.Datasets.iris()

DF.mutate(iris,
  for col <- across( ["sepal_width", "sepal_length", "petal_length", "petal_width"]) do
    {col.name, (col - mean(col)) / variance(col)}
  end
)

Screenshot 2023-08-01 at 22.56.59917×553 44.3 KB

why it has this error?
Thank you in advance