1 files changed, 2 insertions, 0 deletions

diff --git a/environment.scm b/environment.scm
index 4917a40..37f9fa4 100644
--- a/environment.scm
+++ b/environment.scm
@@ -29,6 +29,7 @@
       (error "Too few or too many arguments supplied" vars vals)))
 
 ;;; variable lookup
+(define (is-unassigned? u) (eq? u '*unassigned*))
 (define (lookup-variable-value var env)
   (cond
     [(empty-env? env) (error "Unbound variable" var)]
@@ -76,5 +77,6 @@
                        the-empty-env)])
     (define-variable! 'true #t initial-env)
     (define-variable! 'false #f initial-env)
+    (define-variable! '*unassigned* #f initial-env)
     initial-env))