doc: new buildn example.

* txr.1: Adding breadth-first traversal example showing queue
capability of the list-builder.

1 files changed, 15 insertions, 0 deletions

diff --git a/txr.1 b/txr.1
index 031d7d9e..06cb8305 100644
--- a/txr.1
+++ b/txr.1
@@ -32462,6 +32462,21 @@ macro for deleting a place.
   ;; add function:
 
   (build [mapdo add (range 1 3)]) -> (1 2 3)
+
+  ;; breadth-first traversal of nested list;
+  (defun bf-map (tree visit-fn)
+    (buildn
+      (add tree)
+      (whilet ((item (del)))
+        (if (atom item)
+          [visit-fn item]
+          (each ((el item))
+            (add el))))))
+
+  (let (flat)
+    (bf-map '(1 (2 (3 4 (5))) ((6 7) 8)) (do push @1 flat))
+     (nreverse flat))
+  -> (1 2 8 3 4 6 7 5)
 .brev
 
 .SS* Permutations and Combinations