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diff --git a/test/external/metalang99/examples/duffs_device.c b/test/external/metalang99/examples/duffs_device.c
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+++ b/test/external/metalang99/examples/duffs_device.c
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+/**
+ * Duff's device [1] is a technique to implement loop unrolling through amalgamation of a switch
+ * statement with a do-while loop.
+ *
+ * In this example, we are going to implement automatic generation of Duff's device.
+ *
+ * [1]: https://en.wikipedia.org/wiki/Duff's_device
+ */
+
+#include <metalang99.h>
+
+#include <assert.h>
+
+#define DUFFS_DEVICE(unrolling_factor, counter_ty, count, ...)                                     \
+    do {                                                                                           \
+        if ((count) > 0) {                                                                         \
+            counter_ty DUFFS_DEVICE_n = ((count) + ML99_DEC(unrolling_factor)) / unrolling_factor; \
+            switch ((count) % unrolling_factor) {                                                  \
+            case 0:                                                                                \
+                do {                                                                               \
+                    __VA_ARGS__                                                                    \
+                    ML99_EVAL(ML99_callUneval(genCases, ML99_DEC(unrolling_factor), __VA_ARGS__))  \
+                } while (--DUFFS_DEVICE_n > 0);                                                    \
+            }                                                                                      \
+        }                                                                                          \
+    } while (0)
+
+#define genCases_IMPL(i, ...)                                                                      \
+    ML99_IF(                                                                                       \
+        ML99_NAT_EQ(i, 0),                                                                         \
+        ML99_empty(),                                                                              \
+        ML99_TERMS(                                                                                \
+            v(/* FALLTHROUGH */ case i                                                             \
+              : __VA_ARGS__),                                                                      \
+            ML99_callUneval(genCases, ML99_DEC(i), __VA_ARGS__)))
+
+int main(void) {
+#define ARRAY_LEN        50
+#define UNROLLING_FACTOR 3
+
+    int array[ARRAY_LEN] = { ML99_EVAL(ML99_times(v(ARRAY_LEN), v(5, ))) };
+    int *n_ptr = array;
+
+    // Square all the elements in the array.
+    DUFFS_DEVICE(UNROLLING_FACTOR, int, ARRAY_LEN, {
+        *n_ptr *= *n_ptr;
+        n_ptr++;
+    });
+
+    for (int i = 0; i < ARRAY_LEN; i++) {
+        assert(25 == array[i]);
+    }
+}
+
+/*
+The generated Duff's device:
+
+int DUFFS_DEVICE_n = ((50) + 2) / 3;
+switch ((50) % 3) {
+case 0:
+    do {
+        {
+            *n_ptr *= *n_ptr;
+            n_ptr++;
+        }
+    case 2: {
+        *n_ptr *= *n_ptr;
+        n_ptr++;
+    }
+    case 1: {
+        *n_ptr *= *n_ptr;
+        n_ptr++;
+    }
+    } while (--DUFFS_DEVICE_n > 0);
+}
+*/