clojure.core/merge-with
([f & maps])
  Returns a map that consists of the rest of the maps conj-ed onto
  the first.  If a key occurs in more than one map, the mapping(s)
  from the latter (left-to-right) will be combined with the mapping in
  the result by calling (f val-in-result val-in-latter).

Using merge-with yields a rather terse code, comprising a tree-walker and a final tally.

(defn count-syms
  "count number time of a symbol occurs in the first place of a sequence"
  [obj]
  (if (not (seq? obj))
      {} ; only count sequences
      (do
        (reduce #(merge-with + %1 %2)
          (conj
            (map
                count-syms
                (filter seq? (rest obj)))
            (inc-dict dict (first obj)))))))

sym	count	len
clojure.core/list	3872	17
quote	2749	5
defn	1757	4
clojure.core/seq	1481	16
clojure.core/concat	1480	19
=	1074	1
let	1042	3
is	885	2
.	836	1
if	826	2
defmethod	431	9
def	407	3
complex	377	7
defn-	376	5
fn	356	2
first	346	5
defmacro	298	8
clojure.core/apply	264	18
deftest	257	7
str	252	3
when	244	4
imaginary	238	9
recur	237	5
apply	229	5
instance?	220	9
nil	217	3
and	214	3
next	208	4
cl-format	206	9
fn*	197	3
ns	194	2
map	193	3
:use	187	4
list	184	4
1	181	1
count	178	5
+	169	1
seq	167	3
reduce	165	6
are	148	3
cons	143	4
println	141	7
or	141	2
not	135	3
loop	133	4
thrown?	127	7
*	127	1
conj	125	4
print	124	5
nth	124	3
-	116	1
doseq	114	5

Now, I have a study-plan.

Incidentally, waterfront IDE is quite nice, particularly because I don’t have to learn Emacs key-bindings. There are some moments when it barfs and prints out hashmaps incorrectly, but I enjoyed working in a REPL environment.