diff --git a/networks.rkt b/networks.rkt
index f7c8d68..5f59387 100644
--- a/networks.rkt
+++ b/networks.rkt
@@ -150,7 +150,12 @@
                [sbn? contract?]))
 
 (module+ test
-  (require rackunit))
+  (require rackunit)
+  ;; When this variable is set to #t, some particularly expensive test
+  ;; cases are omitted.
+  (define skip-expensive-tests? #t)
+  (unless skip-expensive-tests?
+    (displayln "Running the complete test suite...")))
 
 
 ;;; =================
@@ -526,23 +531,27 @@
 
 (module+ test
   (test-case "build-interaction-graph"
-    (define n-bool
-      (hash 'x '(not y)
-            'y 'x
-            'z '(and y z)
-            't '(or (and (not x) y)
-                    (and x (not y)))))
-    (define bool-doms (make-boolean-domains '(x y z t)))
-    (check-equal? (graphviz (build-interaction-graph/form n-bool bool-doms))
-                  "digraph G {\n\tnode0 [label=\"y\"];\n\tnode1 [label=\"z\"];\n\tnode2 [label=\"x\"];\n\tnode3 [label=\"t\"];\n\tsubgraph U {\n\t\tedge [dir=none];\n\t\tnode0 -> node2;\n\t\tnode1 -> node1;\n\t}\n\tsubgraph D {\n\t\tnode0 -> node3;\n\t\tnode0 -> node1;\n\t\tnode2 -> node3;\n\t}\n}\n")
-    (define n-multi
-      (hash 'x '(min (+ y 1) 2)
-            'y '(max (- y 1) 0)
-            'z '(- 2 y)
-            't '(abs (- y 1))))
-    (define 123-doms (make-same-domains '(x y z t) '(0 1 2)))
-    (check-equal? (graphviz (build-interaction-graph/form n-multi 123-doms))
-                  "digraph G {\n\tnode0 [label=\"y\"];\n\tnode1 [label=\"z\"];\n\tnode2 [label=\"x\"];\n\tnode3 [label=\"t\"];\n\tsubgraph U {\n\t\tedge [dir=none];\n\t\tnode0 -> node0;\n\t}\n\tsubgraph D {\n\t\tnode0 -> node2;\n\t\tnode0 -> node3;\n\t\tnode0 -> node1;\n\t}\n}\n")))
+    (cond
+      [skip-expensive-tests?
+       (displayln "Skipping test case build-interaction-graph.")]
+      [else
+       (define n-bool
+         (hash 'x '(not y)
+               'y 'x
+               'z '(and y z)
+               't '(or (and (not x) y)
+                       (and x (not y)))))
+       (define bool-doms (make-boolean-domains '(x y z t)))
+       (check-equal? (graphviz (build-interaction-graph/form n-bool bool-doms))
+                     "digraph G {\n\tnode0 [label=\"y\"];\n\tnode1 [label=\"z\"];\n\tnode2 [label=\"x\"];\n\tnode3 [label=\"t\"];\n\tsubgraph U {\n\t\tedge [dir=none];\n\t\tnode0 -> node2;\n\t\tnode1 -> node1;\n\t}\n\tsubgraph D {\n\t\tnode0 -> node3;\n\t\tnode0 -> node1;\n\t\tnode2 -> node3;\n\t}\n}\n")
+       (define n-multi
+         (hash 'x '(min (+ y 1) 2)
+               'y '(max (- y 1) 0)
+               'z '(- 2 y)
+               't '(abs (- y 1))))
+       (define 123-doms (make-same-domains '(x y z t) '(0 1 2)))
+       (check-equal? (graphviz (build-interaction-graph/form n-multi 123-doms))
+                     "digraph G {\n\tnode0 [label=\"y\"];\n\tnode1 [label=\"z\"];\n\tnode2 [label=\"x\"];\n\tnode3 [label=\"t\"];\n\tsubgraph U {\n\t\tedge [dir=none];\n\t\tnode0 -> node0;\n\t}\n\tsubgraph D {\n\t\tnode0 -> node2;\n\t\tnode0 -> node3;\n\t\tnode0 -> node1;\n\t}\n}\n")])))
 
 ;;; Given a network, builds its signed interaction graph.  The graph
 ;;; has variables as nodes and has a directed edge from x to
@@ -564,23 +573,27 @@
 
 (module+ test
   (test-case "build-signed-interaction-graph"
-    (define n-bool
-      (hash 'x '(not y)
-            'y 'x
-            'z '(and y z)
-            't '(or (and (not x) y)
-                    (and x (not y)))))
-    (define bool-doms (make-boolean-domains '(x y z t)))
-    (check-equal? (graphviz (build-signed-interaction-graph/form n-bool bool-doms))
-                  "digraph G {\n\tnode0 [label=\"y\"];\n\tnode1 [label=\"z\"];\n\tnode2 [label=\"x\"];\n\tnode3 [label=\"t\"];\n\tsubgraph U {\n\t\tedge [dir=none];\n\t\tnode1 -> node1 [label=\"1\"];\n\t}\n\tsubgraph D {\n\t\tnode0 -> node2 [label=\"-1\"];\n\t\tnode0 -> node3 [label=\"0\"];\n\t\tnode0 -> node1 [label=\"1\"];\n\t\tnode2 -> node3 [label=\"0\"];\n\t\tnode2 -> node0 [label=\"1\"];\n\t}\n}\n")
-    (define n-multi
-      (hash 'x '(min (+ y 1) 2)
-            'y '(max (- y 1) 0)
-            'z '(- 2 y)
-            't '(abs (- y 1))))
-    (define 123-doms (make-same-domains '(x y z t) '(0 1 2)))
-    (check-equal? (graphviz (build-signed-interaction-graph/form n-multi 123-doms))
-                  "digraph G {\n\tnode0 [label=\"y\"];\n\tnode1 [label=\"z\"];\n\tnode2 [label=\"x\"];\n\tnode3 [label=\"t\"];\n\tsubgraph U {\n\t\tedge [dir=none];\n\t\tnode0 -> node0 [label=\"1\"];\n\t}\n\tsubgraph D {\n\t\tnode0 -> node2 [label=\"1\"];\n\t\tnode0 -> node3 [label=\"0\"];\n\t\tnode0 -> node1 [label=\"-1\"];\n\t}\n}\n")))
+    (cond
+      [skip-expensive-tests?
+       (displayln "Skipping test case build-signed-interaction-graph.")]
+      [else
+       (define n-bool
+         (hash 'x '(not y)
+               'y 'x
+               'z '(and y z)
+               't '(or (and (not x) y)
+                       (and x (not y)))))
+       (define bool-doms (make-boolean-domains '(x y z t)))
+       (check-equal? (graphviz (build-signed-interaction-graph/form n-bool bool-doms))
+                     "digraph G {\n\tnode0 [label=\"y\"];\n\tnode1 [label=\"z\"];\n\tnode2 [label=\"x\"];\n\tnode3 [label=\"t\"];\n\tsubgraph U {\n\t\tedge [dir=none];\n\t\tnode1 -> node1 [label=\"1\"];\n\t}\n\tsubgraph D {\n\t\tnode0 -> node2 [label=\"-1\"];\n\t\tnode0 -> node3 [label=\"0\"];\n\t\tnode0 -> node1 [label=\"1\"];\n\t\tnode2 -> node3 [label=\"0\"];\n\t\tnode2 -> node0 [label=\"1\"];\n\t}\n}\n")
+       (define n-multi
+         (hash 'x '(min (+ y 1) 2)
+               'y '(max (- y 1) 0)
+               'z '(- 2 y)
+               't '(abs (- y 1))))
+       (define 123-doms (make-same-domains '(x y z t) '(0 1 2)))
+       (check-equal? (graphviz (build-signed-interaction-graph/form n-multi 123-doms))
+                     "digraph G {\n\tnode0 [label=\"y\"];\n\tnode1 [label=\"z\"];\n\tnode2 [label=\"x\"];\n\tnode3 [label=\"t\"];\n\tsubgraph U {\n\t\tedge [dir=none];\n\t\tnode0 -> node0 [label=\"1\"];\n\t}\n\tsubgraph D {\n\t\tnode0 -> node2 [label=\"1\"];\n\t\tnode0 -> node3 [label=\"0\"];\n\t\tnode0 -> node1 [label=\"-1\"];\n\t}\n}\n")])))
 
 ;;; ====================
 ;;; Dynamics of networks