add metalang99 testsuite (preprocessor stress testing)

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diff --git a/test/external/metalang99/idioms.md b/test/external/metalang99/idioms.md
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+# Idioms
+
+_This document describes common idioms when using Metalang99, i.e., code patterns that have not been reified into abstractions yet._
+
+## Detecting a keyword followed by parentheses
+
+To detect something like `abracadabra(1, 2, 3)`, follow this simple pattern:
+
+```c
+#define DETECT_ABRACADABRA(x)               ML99_IS_TUPLE(ML99_CAT(DETECT_ABRACADABRA_, x))
+#define DETECT_ABRACADABRA_abracadabra(...) ()
+
+// 1
+DETECT_ABRACADABRA(abracadabra(1, 2, 3))
+
+// 0
+DETECT_ABRACADABRA(blah)
+```
+
+## Extracting a value of a keyword followed by parentheses
+
+To get `1, 2, 3` from `abracadabra(1, 2, 3)`:
+
+```c
+#define EXTRACT_ABRACADABRA(x)               ML99_CAT(EXTRACT_ABRACADABRA_, x)
+#define EXTRACT_ABRACADABRA_abracadabra(...) __VA_ARGS__
+
+// 1, 2, 3
+EXTRACT_ABRACADABRA(abracadabra(1, 2, 3))
+```
+
+## Interspersing a comma
+
+To intersperse a comma between one or more elements, put a comma before each element and pass them all to `ML99_variadicsTail`:
+
+```c
+#define ARRAY_SUBSCRIPTS(array, n)                                                                 \
+    ML99_EVAL(ML99_variadicsTail(ML99_repeat(v(n), ML99_appl(v(GEN_SUBSCRIPT), v(array)))))
+#define GEN_SUBSCRIPT_IMPL(array, i) v(, (array)[i])
+#define GEN_SUBSCRIPT_ARITY          2
+
+// (animals)[0], (animals)[1], (animals)[2]
+ARRAY_SUBSCRIPTS(animals, 3)
+```