.

wcfs/internal/xbtree.py

@@ -140,6 +140,16 @@ class Tree(object):
     def copy(t):
         return Tree(t.keyv, *[_.copy() for _ in t.children])
 
+    # firstbucket returns Bucket reachable through leftmost child links.
+    def firstbucket(t):
+        child0 = t.children[0]
+        if isinstance(child0, Bucket):
+            return child0
+        assert isinstance(child0, Tree)
+        return child0.firstbucket()
+
+
+
 
 # Bucket represents a bucket node.
 class Bucket(object):
@@ -394,7 +404,7 @@ def Restructure(ztree, newStructure):
                     zstate = (zstate,)
 
                     # firstbucket
-                    zstate += (None,)   # XXX
+                    zstate += (node.firstbucket().Z,)
 
             # https://github.com/zopefoundation/BTrees/blob/4.5.0-1-gc8bf24e/BTrees/BucketTemplate.c#L1195
             if isinstance(node, Bucket):