Janet's `case` for multi-arity dispatch: `(defn f [& args] (case (length args) 1 ... 2 ...))`. Used in core-derive, core-isa?, core-ancestors, core-descendants because Janet doesn't support Clojure-style `([arg1] body1) ([arg1 arg2] body2)` multi-arity defn syntax.
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Janet's boolean function doesn't exist — use (if x true false). Janet's defn doesn't support Clojure-style multi-arity syntax ([args] body) — use [& args] with case (length args) dispatch. fn? exists as Janet builtin (not Jolt core fn) — use (or (function? x) (cfunction? x)) in tests.
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Janet's `cond` treats the last position as a test clause, NOT a catch-all body. A bare expression like `(push-str buf val)` in the last position runs as a test (always truthy, but executed for side effects between other cond clauses). Use `true (push-str buf val)` to make it a proper catch-all body. Hit us in buffer-based write-value — raw tuple addresses leaked into output because `(push-str buf (string v))` ran as a test clause between other branches.