task1

defmodule Composition do
  def inc(x) do
    x + 1
  end

  def square(x) do
    x * x
  end

  def double(x) do
    x * 2
  end

  def composition(FILL_IN_1) do
    FILL_IN_2
  end

  def isd do
    FILL_IN_3
  end
end

18 = Composition.isd.(2)
72 = Composition.isd.(5)
42 = Composition.composition([&Composition.double/1]).(21)