Are clojure function cyclic dependencies specifically disallowed by design, or is it just a reader behaviour?

Question

If I do the following in clojure

(defn sub1a [a]
  (cond
    (= a 0) 0
    true (sub1b (- a 1) )))

(defn sub1b [a]
  (cond
    (= a 0) 0
    true (sub1a (- a 1) )))

(println (sub1a 10))

I get the following error:

java.lang.Exception: Unable to resolve symbol: sub1b in this context

But if I do the following:

(defn sub1a [a]
  (cond
    (= a 0) 0
    true (- a 1)))

(defn sub1b [a]
  (cond
    (= a 0) 0
    true (- a 1)))

(defn sub1a [a]
  (cond
    (= a 0) 0
    true (sub1b (- a 1) )))

(defn sub1b [a]
  (cond
    (= a 0) 0
    true (sub1a (- a 1) )))

(println (sub1a 10))

It runs just fine.

Is this by design, or just a function of the way the Clojure reader works?