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