Enum.map_every/3

defmodule Enum do
  def map_every(enumerable, nth, fun)
end

Returns a list of results of invoking fun on every nth element of enumerable, starting with the first element.

The first element is always passed to the given function, unless nth is 0.

The second argument specifying every nth element must be a non-negative integer.

If nth is 0, then enumerable is directly converted to a list, without fun being ever applied.