Add credo to the project and:

- Run mix credo --all to identify possible code optimizations
 - Resolve most of the errors generated by credo such as:
   - Numbers larger than 9999 should be written with underscores: 58_127
   - Modules should have a @moduledoc tag
   - Comparison will always return true

test/koans/comprehensions_koans_test.exs

@@ -1,18 +1,17 @@
-defmodule ComprehensionsTests do
-  use ExUnit.Case
-  import TestHarness
-
-  test "Comprehensions" do
-    answers = [
-      [1, 4, 9, 16],
-      [1, 4, 9, 16],
-      ["Hello World", "Apple Pie"],
-      ["little dogs", "little cats", "big dogs", "big cats"],
-      [4, 5, 6],
-      %{"Pecan" => "Pecan Pie", "Pumpkin" => "Pumpkin Pie"}
-    ]
-
-    test_all(Comprehensions, answers)
-  end
-
-end
+defmodule ComprehensionsTests do
+  use ExUnit.Case
+  import TestHarness
+
+  test "Comprehensions" do
+    answers = [
+      [1, 4, 9, 16],
+      [1, 4, 9, 16],
+      ["Hello World", "Apple Pie"],
+      ["little dogs", "little cats", "big dogs", "big cats"],
+      [4, 5, 6],
+      %{"Pecan" => "Pecan Pie", "Pumpkin" => "Pumpkin Pie"}
+    ]
+
+    test_all(Comprehensions, answers)
+  end
+end