Comparing lists for equal/sublist/superlist

sublist.exs

defmodule Sublist do

  def compare(a,b) when a === b, do: :equal
  def compare(a,b) do
    cond do
      a === b ->       :equal
      sublist?(a,b) -> :sublist
      sublist?(b,a) -> :superlist
      true ->          :unequal
    end
  end

  defp sublist?([],_), do: false
  defp sublist?(_,[]), do: false
  defp sublist?(first, second=[_|tail]) do
    if at_head?(first, second) do
      true
    else
      sublist?(first, tail)
    end
  end

  defp at_head?([],_), do: true
  defp at_head?(_,[]), do: false

  defp at_head?([a|restA], [a|restB]) do
    at_head?(restA,restB)
  end

  defp at_head?(_,_), do: false
end

ExUnit.start

defmodule SublistTest do
  use ExUnit.Case, async: true

  test "A is a sublist of B(1)" do
    assert :sublist == Sublist.compare([1,2,3], [1,2,3,4,5])
  end

  test "A is a sublist of B(2)" do
    assert :sublist == Sublist.compare([3,4,5], [1,2,3,4,5])
  end

  test "A is a sublist of B(3)" do
    assert :sublist == Sublist.compare([3,4], [1,2,3,4,5])
  end

  test "A = B" do
    assert :equal == Sublist.compare([1,2,3], [1,2,3])
  end

  test "A is a superset of B" do
    assert :superlist == Sublist.compare([1,2,3,4,5], [2,3,4])
  end

  test "A is not a superset of B" do
    assert :unequal == Sublist.compare([1,2,4], [1,2,3,4,5])
  end

  test "A is not a subset of B" do
    assert :unequal == Sublist.compare([1,2,3,4,5], [1,2,4])
  end

  test "A is not equal to B" do
    assert :unequal == Sublist.compare([1,2,4], [1,2,3,4,5])
  end

  test "Empty list is in B" do
    assert :unequal == Sublist.compare([],[1,2,3])
  end

  test "A is not in empty" do
    assert :unequal == Sublist.compare([1,2,3], [])
  end
end