example: Add the section on TBF.

example/example.org

@@ -688,6 +688,84 @@ tab
    the - arcs in the graph, as it does not have access to the symbolic
    description of the function.
 
+** Threshold Boolean functions (TBF)
+ =dds= includes some useful definitions for working with threshold
+   Boolean functions (TBF).  A TBF is defined as a vector of weights
+   and a threshold.  For example, the following defines a
+   TBF implementing the logical AND.
+   #+BEGIN_SRC racket :results output drawer
+(tbf #(1 1) 1)
+   #+END_SRC
+
+   #+RESULTS:
+   :RESULTS:
+   (tbf '#(1 1) 1)
+   :END:
+
+   This TBF only returns 1 when both inputs are activated, which
+   brings their weighted some above 1: 1 \cdot 1 + 1 \cdot 1 = 2 > 1.
+   #+BEGIN_SRC racket :results output drawer
+(apply-tbf (tbf #(1 1) 1) #(1 1))
+   #+END_SRC
+
+   #+RESULTS:
+   :RESULTS:
+   1
+   :END:
+
+   Let's actually check out the truth table of this TBF.
+   #+BEGIN_SRC racket :results table drawer
+(tbf-tabulate (tbf #(1 1) 1))
+   #+END_SRC
+
+   #+RESULTS:
+   :RESULTS:
+   | 0 | 0 | 0 |
+   | 0 | 1 | 0 |
+   | 1 | 0 | 0 |
+   | 1 | 1 | 1 |
+   :END:
+
+   This truth table corresponds indeed to the logical AND.
+
+ =dds= allows reading TBFs from Org tables.  In this case, the last
+   column in each row is treated as the threshold, while the first
+   values are taken to be the weights.  Consider, for example, the
+   following table:
+   #+NAME: simple-tbf
+   | 1 | 1 | 0 |
+   | 1 | 1 | 1 |
+
+   You can read the two TBFs defined in this table in the following
+   way:
+   #+BEGIN_SRC racket :results output drawer :var simple-tbf=munch-sexp(simple-tbf)
+(read-org-tbfs simple-tbf)
+   #+END_SRC
+
+   #+RESULTS:
+   :RESULTS:
+   (list (tbf '#(1 1) 0) (tbf '#(1 1) 1))
+   :END:
+
+   The first TBF implements the logical OR of its inputs, while the
+   second TBF implements the logical AND.  Let's check it by
+   tabulating both functions:
+   #+BEGIN_SRC racket :results table drawer :var simple-tbf=munch-sexp(simple-tbf)
+(tbf-tabulate* (read-org-tbfs simple-tbf))
+   #+END_SRC
+
+   #+RESULTS:
+   :RESULTS:
+   | 0 | 0 | 0 | 0 |
+   | 0 | 1 | 1 | 0 |
+   | 1 | 0 | 1 | 0 |
+   | 1 | 1 | 1 | 1 |
+   :END:
+
+   The first two columns of this table give the values of the two
+   inputs.  The third column gives the values of the first TBF, and
+   the fourth column gives the values of the second TBF.
+
 * Reaction systems
   :PROPERTIES:
   :header-args:racket: :prologue "#lang racket\n(require graph dds/rs dds/utils)"