diff --git a/scribblings/utils.scrbl b/scribblings/utils.scrbl
index b2e412b..06c1b60 100644
--- a/scribblings/utils.scrbl
+++ b/scribblings/utils.scrbl
@@ -17,6 +17,21 @@ the package: evaluating sexps, manipulating lists,
 		 [sandbox-memory-limit 50])
    (make-evaluator 'racket/base #:requires '("utils.rkt"))))
 
+@section{Base types}
+
+@defidform[Variable]{
+
+Any Racket symbol.  Designates a variable in a discrete dynamical network.
+
+}
+
+@defform[(VariableMapping A)]{
+
+An immutable mapping (a hash table) assigning elements of type @racket[A] to
+the variables.
+
+}
+
 @section{Hashtable injection}
 
 This section defines some utilities to streamline the usage of hash tables
diff --git a/utils.rkt b/utils.rkt
index 6f082ac..cc7ba79 100644
--- a/utils.rkt
+++ b/utils.rkt
@@ -15,12 +15,11 @@
            (for-syntax syntax/parse racket/list))
   (provide (all-defined-out))
 
-  (define a 1))
+  (define-type Variable Symbol)
+  (define-type (VariableMapping A) (Immutable-HashTable Variable A)))
 
 (require 'typed)
 
-(displayln a)
-
 ;;; Untyped section.
 
 (provide