I pushed Utils out. It features reduce-by but also variations on cond (the ones introduced in the comments of A Flatter Cond) and if-let.

cond supports the following operators: :let, :when, :when-let and vectors in test position are treated as the binding form to a if-let (thus setting the local bindings) for the “then” expression not for the rest of the cond.

if-let supports :let to introduce new bindings without testing their expression for truth.

One notable twist to these macros is that thanks to net.cgrand.xmacros they are extensible: new operators can safely be participated to the macros (proper namespacing is enforced). See the cond implementation, abbreviated below:

(defmacro cond
 "An extensible variation on cond. Trailing else is supported.
keywords in test position are treated as operators.

Operators impls signature is [rhs else] where rhs is the \"then\"
expression next to the operator."
  [& clauses]
  (when-let [[op rhs & more-clauses] (seq clauses)]
    (if (next clauses)
      (x/expand-op op rhs `(cond ~@more-clauses))
      test)))

(x/defdefault-op cond [test-expr then else]
  (if (vector? test-expr)
    `(if-let ~test-expr ~then ~else)
    `(if ~test-expr ~then ~else)))

(x/defop cond :let
  "Introduces local bindings."
  [bindings cont]
  `(let ~bindings ~cont))

(x/defop cond :when
  "Short-circuits the rest of the cond if false"
  [test-expr cont]
  `(when ~test-expr ~cont))

(x/defop cond :when-let
 "Short-circuits the rest of the cond if false and introduces local
bindings."
  [bindings cont]
  `(when-let ~bindings ~cont))

I recently published Megaref a refined (and working) version of the idea described in A World in a Ref.

New and notesworthy:

• actual working code,
• the concurrency (number of guards) can be tuned at runtime,
• tuning happens through an uniform interface (get-options, set-option!),
• subrefs (and backward compatible replacements for usual STM functions) allow to easily migrate “polyref” code to megaref (see the conversion of Rich Hickey’s ant demo — mind the typo in the commit message).

Feedback welcome!