Move parameter guarding into its own module

engine/guards.scm

@@ -0,0 +1,13 @@
+(module (engine guards) ()
+(import scheme
+	(chicken base)
+	(chicken module))
+
+;; Utility function for guarding parameter values
+(export guarded-parameter)
+(define (guarded-parameter default predicate)
+  (make-parameter default (lambda (val)
+			    (if (predicate val)
+				val
+				default))))
+)