diff --git a/assessments/semiprimes/server/lib/extras.ex b/assessments/semiprimes/server/lib/extras.ex
new file mode 100644
index 000000000..4d2a938fc
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+++ b/assessments/semiprimes/server/lib/extras.ex
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+defmodule Extras do
+  @doc """
+  Return an ascending range starting at `a` and ending at `b` (exclusive).
+
+  ## Examples
+
+      iex> Extras.range(2, 5)
+      [2, 3, 4]
+
+  """
+  def range(a, b) do
+    if b <= a do
+      []
+    else
+      [a] ++ range(a + 1, b)
+    end
+  end
+end
diff --git a/assessments/semiprimes/server/test/extras_test.exs b/assessments/semiprimes/server/test/extras_test.exs
new file mode 100644
index 000000000..67d0b8875
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+++ b/assessments/semiprimes/server/test/extras_test.exs
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+defmodule ExtrasTest do
+  use ExUnit.Case
+  doctest Extras
+
+  describe "range" do
+    test "returns an empty list for descending sequences" do
+      assert Extras.range(0, -2) == []
+    end
+
+    test "returns an empty list for non-ascending sequences" do
+      assert Extras.range(8, 8) == []
+    end
+
+    test "returns an exclusive range" do
+      assert Extras.range(3, 6) == [3, 4, 5]
+    end
+  end
+end