diff --git a/wasp/parser/box.py b/wasp/parser/box.py
index 30ea671..5ba61ca 100644
--- a/wasp/parser/box.py
+++ b/wasp/parser/box.py
@@ -28,11 +28,11 @@ class Pair(BaseBox):
             y = self.y
         return "(%s . %s)" % (self.x, y)
 
-    def r_cdr(self, cdr):
+    def _r_cdr(self, cdr):
         if self.y is None:
             return (self.x.ast(), )
         else:
-            return cdr + (self.x.ast(), ) + self.y.r_cdr(cdr)
+            return cdr + (self.x.ast(), ) + self.y._r_cdr(cdr)
 
     def ast(self):
         car = self.x.ast()
@@ -40,7 +40,7 @@ class Pair(BaseBox):
         if self.y is None:
             cdr = None
         elif type(self.y) is Pair:
-            cdr = list(self.y.r_cdr(()))
+            cdr = list(self.y._r_cdr(()))
         else:
             cdr = self.y.ast()