Add reaction and Reaction.

rs.rkt

@@ -4,12 +4,18 @@
   (require typed/graph "utils.rkt" "dynamics.rkt")
 
   (provide
-   Species)
+   Species (struct-out reaction) Reaction)
 
   (module+ test
     (require typed/rackunit))
 
   (define-type Species Symbol)
+
+  (struct reaction ([reactants : (Setof Species)]
+                    [inhibitors : (Setof Species)]
+                    [products : (Setof Species)])
+    #:transparent
+    #:type-name Reaction)
   )
 
 (require graph "utils.rkt" "generic.rkt")

scribblings/rs.scrbl

@@ -32,6 +32,21 @@ A synonym of @racket[Symbol].
 
 }
 
+@defstruct*[reaction ([reactants (Setof Species)]
+		      [inhibitors (Setof Species)]
+		      [products (Setof Species)])]{
+
+A reaction is a triple of sets, giving the reactants, the inhibitors,
+and the products, respectively.
+
+}
+
+@deftype[Reaction]{
+
+The type of the instances of @racket[reaction].
+
+}
+
 @section{Org-mode interaction}
 
 This section contains some useful primitives for