Enum.flat_map([:a, :b, :c], fn x -> [x, x] end)
[:a, :a, :b, :b, :c, :c]

iex> Enum.flat_map([{1, 3}, {4, 6}], fn {x, y} -> x..y end)
[1, 2, 3, 4, 5, 6]

iex> Enum.flat_map([:a, :b, :c], fn x -> [[x]] end)
[[:a], [:b], [:c]]