In section “Managing Per-Thread State with Vars” it explains clojure’s set! special form (p. 173). It explains the usage of set! inside clojure.xml, but omits that it must be inside a binding context, because if the current value of the var is the root binding, it will fail.

So, the actual usage inside clojure.xml is:

(binding [*stack* nil
          *current* (struct element)
          *state* :between
          *sb* nil]
  ; After this point we can use set! on the vars inside the binding.
  ; Otherwise set! will print
  ; Can't change/establish root binding of: the-var-I-want-to-set with set
  (startparse s content-handler)
  ((:content *current*) 0))