New list-builder method: oust.

* autoload.c (build_set_entries): Add oust symbol.

* stdlib/build.tl (list-builder postinit): Call the self
argument self instead of bc, for consistency with other
methods.
(list-builder oust): New method.
(list-builder-flets): Add local function oust.

* tests/012/seq.tl: New tests.

* txr.1: Documented.

* stdlib/doc-syms.tl: Updated.

5 files changed, 70 insertions, 6 deletions

diff --git a/autoload.c b/autoload.c
index bfe53a51..f2ecfb11 100644
--- a/autoload.c
+++ b/autoload.c
@@ -373,7 +373,7 @@ static val build_set_entries(val fun)
   val name_noload[] = {
     lit("head"), lit("tail"), lit("add"), lit("add*"), lit("pend"),
     lit("pend*"), lit("ncon"), lit("ncon*"), lit("get"),
-    lit("del"), lit("del*"),
+    lit("del"), lit("del*"), lit("oust"),
     nil
   };
   autoload_set(al_struct, sname, fun);
diff --git a/stdlib/build.tl b/stdlib/build.tl
index 3837f43f..e5cba268 100644
--- a/stdlib/build.tl
+++ b/stdlib/build.tl
@@ -28,9 +28,16 @@
 (defstruct list-builder ()
   head tail
 
-  (:postinit (bc)
-    (set bc.head (cons nil bc.head)
-         bc.tail bc.head))
+  (:postinit (self)
+    (set self.head (cons nil self.head)
+         self.tail self.head))
+
+  (:method oust (self . lists)
+    (if lists
+      (let ((nl [apply append lists]))
+        (set self.tail (usr:rplacd self.head nl)))
+      (set self.tail (usr:rplacd self.head nil)))
+    self)
 
   (:method add (self . items)
     (let ((st self.tail))
@@ -112,7 +119,7 @@
 
 (defun sys:list-builder-flets (lb-form)
   (nconc
-    (collect-each ((op '(add add* pend pend* ncon ncon*)))
+    (collect-each ((op '(add add* pend pend* ncon ncon* oust)))
       ^(,op (. args)
          (qref ,lb-form (,op . args))
          nil))
diff --git a/stdlib/doc-syms.tl b/stdlib/doc-syms.tl
index 7a9348ee..95092836 100644
--- a/stdlib/doc-syms.tl
+++ b/stdlib/doc-syms.tl
@@ -1391,6 +1391,7 @@
       ("orec" "N-0003ED2C")
       ("orf" "N-01E7D2AD")
       ("ors" "N-02D33A3D")
+      ("oust" "N-0126D3FF")
       ("output" "N-0159EBE7")
       ("package-alist" "N-017F684C")
       ("package-fallback-list" "N-027A535C")
diff --git a/tests/012/seq.tl b/tests/012/seq.tl
index 1706e6df..ae42a13e 100644
--- a/tests/012/seq.tl
+++ b/tests/012/seq.tl
@@ -13,6 +13,11 @@
 (test (build (add 1) (add 2) (pend (get) (get))) (1 2 1 2 1 2))
 (test (build (add 1) (add 2) (pend* (get) (get))) (1 2 1 2 1 2))
 
+(mtest
+  (build (add 1 2) (oust)) nil
+  (build (add 1 2) (oust '(3 4)) (add 5)) (3 4 5)
+  (build (add 1 2) (oust '(3 4) '(5)) (add 6)) (3 4 5 6))
+
 (set *print-circle* t)
 
 (stest (build (add 1) (add 2) (ncon (get))) "#1=(1 2 . #1#)")
diff --git a/txr.1 b/txr.1
index ac7abd73..f1541ceb 100644
--- a/txr.1
+++ b/txr.1
@@ -38037,7 +38037,7 @@ methods extend the list being constructed by a
 object by adding lists to it. The
 .code pend
 method catenates the
-.code list
+.meta list
 arguments together as if by the
 .code append
 function, then appends the resulting list to
@@ -38146,6 +38146,56 @@ object.
   -> (1 2 3 4 . 5)
 .brev
 
+.coNP Method @ oust
+.synb
+.mets << list-builder .(oust << list *)
+.syne
+.desc
+The
+.code oust
+method discards the list constructed so far, optionally
+replacing it with new material.
+
+The
+.code oust
+method catenates the
+.meta list
+arguments together as if by the
+.code append
+function. The resulting list, which is empty
+if there are no
+.meta list
+arguments, then replaces the object's
+list constructed so far.
+
+The
+.code oust
+method returns the
+.meta list-builder
+object.
+
+.TP* Examples:
+
+.verb
+  ;; Build the list (3 4) by first building (1 2),
+  ;; then discarding that and adding 3 and 4:
+
+  (let ((lb (build-list)))
+    lb.(add 1 2)
+    lb.(oust)
+    lb.(add 3 4)
+    lb.(get))
+  -> (3 4)
+
+  ;; Build the list (3 4 5 6) by first building (1 2),
+  ;; then replacing with catenation of (3 4) and (5 6):
+  (let ((lb (build-list)))
+    lb.(pend '(1 2))
+    lb.(oust '(3 4) '(5 6))
+    lb.(get))
+  -> (3 4 5 6)
+.brev
+
 .coNP Method @ get
 .synb
 .mets << list-builder .(get)
@@ -38238,6 +38288,7 @@ This lexical environment also provides local functions named
 .codn pend* ,
 .codn ncon ,
 .codn ncon* ,
+.codn oust ,
 .codn get ,
 .code del
 and