From 701d5ff8c6a2d4ca6023be345faf4f085db6c689 Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Tue, 21 Jul 2015 20:37:59 -0700
Subject: * share/txr/stdlib/place.tl (defplace cdr): Change deletion semantics
 so that (del (cdr x)) is symmetric with (del (car x)).

* txr.1: Update documentation.
---
 ChangeLog                 |  7 +++++++
 share/txr/stdlib/place.tl |  6 +++++-
 txr.1                     | 36 +++++++++++++++++++++++++++++-------
 3 files changed, 41 insertions(+), 8 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index ea191850..b15a3860 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2015-07-21  Kaz Kylheku  <kaz@kylheku.com>
+
+	* share/txr/stdlib/place.tl (defplace cdr): Change deletion
+	semantics so that (del (cdr x)) is symmetric with (del (car x)).
+
+	* txr.1: Update documentation.
+
 2015-07-21  Kaz Kylheku  <kaz@kylheku.com>
 
 	* eval.c (eval_init): Register new split function.
diff --git a/share/txr/stdlib/place.tl b/share/txr/stdlib/place.tl
index f7e468cf..1d748d99 100644
--- a/share/txr/stdlib/place.tl
+++ b/share/txr/stdlib/place.tl
@@ -327,7 +327,11 @@
     ^(macrolet ((,ssetter (val) ^(sys:rplacd ,',cell ,val)))
        ,body))
   (deleter
-    ^(macrolet ((,deleter () ^(zap (cdr ,',cell))))
+    ^(macrolet ((,deleter ()
+                  (with-gensyms (tmp)
+                    (with-update-expander (cgetter csetter) ',cell nil
+                      ^(let ((,tmp (,cgetter)))
+                         (prog1 (cdr ,tmp) (,csetter (car ,tmp))))))))
        ,body)))
 
  (defplace (vecref vector index :whole args) body
diff --git a/txr.1 b/txr.1
index c7e68ceb..7123f898 100644
--- a/txr.1
+++ b/txr.1
@@ -13499,6 +13499,20 @@ because
 .code pop
 is defined on an empty list.
 
+The abstract concept behind deleting a
+.code car
+is that physically deleting this field from a cons,
+thereby breaking it in half, would result in just the
+.code cdr
+remaining. Though fragmenting a cons in this manner is impossible,
+deletion simulates it by replacing the place which previously held the
+cons, with that cons'
+.code cdr
+field. This semantics happens to coincide with deleting the first element
+of a list by a
+.code pop
+operation.
+
 .coNP Accessors @ cdr and @ rest
 .synb
 .mets (cdr << object )
@@ -13563,18 +13577,26 @@ and is modifiable.
 
 A
 .code cdr
-place supports deletion, according to the following equivalence:
+place supports deletion, according to the following near equivalence:
 
 .cblk
-  (del (cdr obj)) <--> (zap (cdr obj))
+  (del (cdr place)) <--> (prog1 (cdr place)
+                                (set place (car place)))
 .cble
 
-Note that this is different from the delete semantics of
+Of course,
+.code place
+is evaluated only once.
+
+Note that this is symmetric with the delete semantics of
 .code car
-in that
-.code obj
-is not required to be a place, and must not be
-.codn nil .
+in that the cons stored in
+.code place
+goes away, as does the
+.code cdr
+field, leaving just the
+.codn car ,
+which takes the place of the original cons.
 
 .TP*
 Example:
-- 
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