example: Add some text about SBFs.

example/example.org

@@ -766,6 +766,33 @@ tab
    inputs.  The third column gives the values of the first TBF, and
    the fourth column gives the values of the second TBF.
 
+   =dds= also provides a couple shortcuts to deal with SBF—sign
+   Boolean functions.  SBF are TBF with threshold equal to 0:
+   #+BEGIN_SRC racket :results output drawer
+(sbf? (tbf #(1 1) 1))
+(sbf? (tbf #(1 1) 0))
+   #+END_SRC
+
+   #+RESULTS:
+   :RESULTS:
+   #f
+   #t
+   :END:
+
+   You can read SBFs from Org tables, like TBFs:
+   #+NAME: some-sbfs
+   | 1 | -1 |
+   | 2 |  2 |
+
+   #+BEGIN_SRC racket :results output drawer :var some-sbfs=munch-sexp(some-sbfs)
+(read-org-sbfs some-sbfs)
+   #+END_SRC
+
+   #+RESULTS:
+   :RESULTS:
+   (list (tbf '#(1 -1) 0) (tbf '#(2 2) 0))
+   :END:
+
 * Reaction systems
   :PROPERTIES:
   :header-args:racket: :prologue "#lang racket\n(require graph dds/rs dds/utils)"