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  ;; The key concept to understand here is that transients are 
  ;; not meant to be `bashed in place`; always use the value 
  ;; returned by either assoc! or other functions that operate
  ;; on transients.
  
  (defn merge2
    "An example implementation of `merge` using transients."
    [x y]
    (persistent! (reduce
                  (fn [res [k v]] (assoc! res k v))
                  (transient x)
                  y)))
  
  ;; Why always use the return value, and not the original?  Because the return
  ;; value might be a different object than the original.  The implementation
  ;; of Clojure transients in some cases changes the internal representation
  ;; of a transient collection (e.g. when it reaches a certain size).  In such
  ;; cases, if you continue to try modifying the original object, the results
  ;; will be incorrect.
  
  ;; Think of transients like persistent collections in how you write code to
  ;; update them, except unlike persistent collections, the original collection
  ;; you passed in should be treated as having an undefined value.  Only the return
  ;; value is predictable.

  ;; The key concept to understand here is that transients are ;; not meant to be `bashed in place`; always use the value ;; returned by either assoc! or other functions that operate ;; on transients. (defn merge2 "An example implementation of `merge` using transients." [x y] (persistent! (reduce (fn [res [k v]] (assoc! res k v)) (transient x) y))) ;; Why always use the return value, and not the original? Because the return ;; value might be a different object than the original. The implementation ;; of Clojure transients in some cases changes the internal representation ;; of a transient collection (e.g. when it reaches a certain size). In such ;; cases, if you continue to try modifying the original object, the results ;; will be incorrect. ;; Think of transients like persistent collections in how you write code to ;; update them, except unlike persistent collections, the original collection ;; you passed in should be treated as having an undefined value. Only the return ;; value is predictable.