Interesting! Unit tests show the bug is not in merge-rules, as thought,

but in add-rule.

src/milkwood_clj/analyse.clj

@@ -23,7 +23,6 @@
    rules: a rule tree (i.e. a recursively nested map token => rule-tree);
    path: a flat sequence of tokens."
   [rules path]
-  (prn  "Rule: " path)
   (cond
    ;; if we have no more path, we're done.
    (empty? path) nil

test/milkwood_clj/analyse_test.clj

@@ -0,0 +1,39 @@
+(ns milkwood-clj.analyse-test
+  (:require [clojure.test :refer :all]
+            [milkwood-clj.analyse :refer :all]))
+
+(deftest flat-rule-test
+  (testing "Create a rule set with a single rule"
+    (is (= (add-rule nil '("a" "b" "c" "d")) '{"a" {"b" {"c" {"d" nil}}}}))))
+
+(deftest two-distinct-rule-test
+  (testing "Create a set of two rules with no common tokens"
+    (is
+     (=
+      (add-rule (add-rule nil '("a" "b" "c" "d")) '("a" "b" "e" "f"))
+      '{"a" {"b" {"c" {"d" nil} "e" {"f" nil}}}}))))
+
+(deftest two-fork-rule-test
+  (testing "Create a set of two rules with common initial tokens"
+    (is
+     (=
+      (add-rule (add-rule nil '("a" "b" "c" "d")) '("p" "q" "r" "s"))
+      '{"a" {"b" {"c" {"d" nil}}} "p" {"q" {"r" {"s" nil}}}}))))
+
+(deftest merge-two-rules
+  (testing "Merge two rule trees each comprising a single rule, with no tokens common between trees"
+    (is
+     (=
+      (merge-rules
+       (add-rule nil '("a" "b" "c" "d"))
+       (add-rule nil '("p" "q" "r" "s")))
+      '{"a" {"b" {"c" {"d" nil}}} "p" {"q" {"r" {"s" nil}}}}))))
+
+(deftest merge-fork-rules
+  (testing "Merge two rule tees each comprising a single rule, with common initial tokens"
+    (is
+     (=
+      (merge-rules
+       (add-rule nil '("a" "b" "c" "d"))
+       (add-rule nil '("a" "b" "e" "f")))
+      '{"a" {"b" {"c" {"d" nil} "e" {"f" nil}}}}))))

test/milkwood_clj/core_test.clj

@@ -0,0 +1,7 @@
+(ns milkwood-clj.core-test
+  (:require [clojure.test :refer :all]
+            [milkwood-clj.core :refer :all]))
+
+(deftest a-test
+  (testing "Nothing worth testing in core, yet."
+    (is (= 1 1))))