diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..f2c7267
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "tests/extlib/doctest"]
+	path = tests/extlib/doctest
+	url = https://github.com/doctest/doctest.git
diff --git a/tests/extlib/doctest b/tests/extlib/doctest
new file mode 160000
index 0000000..ae7a135
--- /dev/null
+++ b/tests/extlib/doctest
@@ -0,0 +1 @@
+Subproject commit ae7a13539fb71f270b87eb2e874fbac80bc8dda2