diff --git a/csp/constraint.rkt b/csp/constraint.rkt
index 0bf3381c..1ed991de 100644
--- a/csp/constraint.rkt
+++ b/csp/constraint.rkt
@@ -29,8 +29,8 @@
         (define variable (car variables))
         (define domain (hash-ref domains variable))
         (set-field! _list domain
-                    (for/fold ([domain-values (send domain get-values)]) 
-                      ([value (in-list (send domain get-values))] 
+                    (for/fold ([domain-values (domain)]) 
+                      ([value (in-list (domain))] 
                        #:when (not (call variables domains (make-hash (list (cons variable value))))))
                       (remove value domain-values)))        
         (set! constraints (remove (list this variables) constraints))
@@ -48,12 +48,12 @@
         [(= (length unassigned-variables) 1)
          (define unassigned-variable (car unassigned-variables))
          (define unassigned-variable-domain (hash-ref domains unassigned-variable))
-         (for ([value (in-list (send unassigned-variable-domain get-values))])
+         (for ([value (in-list (unassigned-variable-domain))])
            (hash-set! assignments unassigned-variable value)
            (when (not (call variables domains assignments))
              (send unassigned-variable-domain hide-value value)))
          (hash-remove! assignments unassigned-variable)
-         (not (send unassigned-variable-domain values-empty?))] ; if domain had no remaining values, the constraint will be impossible to meet, so return #f
+         (not (null? unassigned-variable-domain))] ; if domain had no remaining values, the constraint will be impossible to meet, so return #f
         [else #t]))
     ))
 
@@ -94,9 +94,9 @@
         [(and forward-check?
               (for*/or ([unassigned-var-domain (in-list (map (λ(uv) (hash-ref domains uv)) unassigned-vars))]
                         [assigned-value (in-list assigned-values)]
-                        #:when (send unassigned-var-domain contains-value? assigned-value))
+                        #:when (member assigned-value (unassigned-var-domain)))
                 (send unassigned-var-domain hide-value assigned-value)
-                (send unassigned-var-domain values-empty?))) #f] ; if domain had no remaining values, the constraint will be impossible to meet, so return #f
+                (null? unassigned-var-domain))) #f] ; if domain had no remaining values, the constraint will be impossible to meet, so return #f
         [else #t]))))
 
 (define all-different-constraint%? (is-a?/c all-different-constraint%))
@@ -125,10 +125,10 @@
           (for ([variable (in-list variables)])
             (when (not (variable . in? . assignments))
               (set! domain (hash-ref domains variable))
-              (when (not (singlevalue . in? . (send domain get-values)))
+              (when (not (singlevalue . in? . (domain)))
                 (set! return-value #f)
                 (return-k))
-              (for ([value (in-list (send domain get-values))])
+              (for ([value (in-list (domain))])
                 (when (not (equal? value singlevalue))
                   (send domain hide-value value))))))
         (set! return-value #t)
diff --git a/csp/domain.rkt b/csp/domain.rkt
index 3242939e..a9063e6f 100644
--- a/csp/domain.rkt
+++ b/csp/domain.rkt
@@ -2,12 +2,18 @@
 (require racket/class racket/list "helper.rkt")
 (provide (all-defined-out))
 
-(define domain%
-  ;; Class used to control possible values for variables
-  ;; When list or tuples are used as domains, they are automatically
-  ;; converted to an instance of that class.
-  
-  (class* object% (printable<%>)
+(define proc<%>
+  (interface* ()
+              ([prop:procedure
+                (λ(this)
+                  (send this get-values))])
+              get-values))
+
+;; Class used to control possible values for variables
+;; When list or tuples are used as domains, they are automatically
+;; converted to an instance of that class.
+(define domain%  
+  (class* object% (printable<%> proc<%>)
     (super-new)
     (init-field set)
     (field [_list set][_hidden null][_states null])
@@ -51,12 +57,6 @@
     (define/public (get-values)
       _list)
     
-    (define/public (values-empty?)
-      (null? _list))
-    
-    (define/public (contains-value? value)
-      (member value _list))
-    
     (define/public (domain-pop!)
       (py-pop! _list))
     
@@ -64,9 +64,7 @@
       (define copied-domain (new domain% [set _list]))
       (set-field! _hidden copied-domain _hidden)
       (set-field! _states copied-domain _states)
-      copied-domain)
-    
-    
-    ))
+      copied-domain)))
+
 (define domain%? (is-a?/c domain%))
 
diff --git a/csp/problem.rkt b/csp/problem.rkt
index d94e8459..852b795a 100644
--- a/csp/problem.rkt
+++ b/csp/problem.rkt
@@ -3,9 +3,8 @@
 (require "domain.rkt" "helper.rkt" "constraint.rkt" "solver.rkt")
 (provide (all-defined-out))
 
-(define/contract problem%
-  ;; Class used to define a problem and retrieve solutions
-  
+;; Class used to define a problem and retrieve solutions
+(define/contract problem%  
   (class/c [reset (->m void?)]
            [set-solver (solver%? . ->m . void?)] 
            [get-solver (->m solver%?)]
@@ -52,7 +51,7 @@
       (define domain (if (domain%? domain-or-values)
                          (send domain-or-values copy)
                          (new domain% [set domain-or-values])))
-      (when (send domain values-empty?) 
+      (when (null? (domain)) 
         (error 'add-variable "domain value is null"))
       (hash-set! _variable-domains variable domain))
     
@@ -114,6 +113,6 @@
       
       (if (for/or ([domain (in-hash-values variable-domains)])
             (send domain reset-state)
-            (send domain values-empty?))
+            (null? (domain)))
           (values null null null)
           (values variable-domains constraints vconstraints)))))
\ No newline at end of file