Enum.flat_map beginners question

[p.t.peter]

Hello, why does it do this?

iex(2)> Enum.flat_map [5,6,7,8,9], &([[&1],[&1]])
[[5], [5], [6], [6], ~c"\a", ~c"\a", ~c"\b", ~c"\b", ~c"\t", ~c"\t"]
iex(3)>