3 files changed, 41 insertions, 4 deletions

diff --git a/chain/src/item/default/extract.rs b/chain/src/item/default/extract.rs
index 96f5119..b99c541 100644
--- a/chain/src/item/default/extract.rs
+++ b/chain/src/item/default/extract.rs
@@ -188,11 +188,25 @@ impl DefaultForest<ForestLabel<GrammarLabel>> {
                 1 => {
                     let child = *roots_stack.first().unwrap();
 
-                    builder.add_vertex(self.vertex_label(child)?.ok_or(Error::NodeNoLabel(child))?);
+                    // Make this a plain node.
+                    //
+                    // This situation will be handled properly in
+                    // later codes.
+
+                    let plain_child_label = ForestLabel::new(
+                        self.vertex_label(child)?
+                            .ok_or(Error::NodeNoLabel(child))?
+                            .label(),
+                        ForestLabelType::Plain,
+                    );
+
+                    builder.add_vertex(plain_child_label);
                 }
                 _ => {
                     let builder_root = builder.add_vertex(original_root_label);
 
+                    // The clone indices have to be preserved so that
+                    // we can track those nodes later.
                     for child in roots_stack.iter().copied() {
                         let child_node = builder.add_vertex(
                             self.vertex_label(child)?.ok_or(Error::NodeNoLabel(child))?,
diff --git a/chain/src/item/default/mod.rs b/chain/src/item/default/mod.rs
index 0781dd2..7369c6f 100644
--- a/chain/src/item/default/mod.rs
+++ b/chain/src/item/default/mod.rs
@@ -1,3 +1,4 @@
+#![debugger_visualizer(gdb_script_file = "printer.py")]
 //! This module provides a default implementation of iten derivation
 //! forest.
 
@@ -72,6 +73,8 @@ impl From<ForestLabelError> for Error {
     }
 }
 
+// FIXME: Provide a better Debug implementation.
+
 /// A default implementation of forest.
 #[derive(Debug, Clone, Graph)]
 pub struct DefaultForest<T: GraphLabel> {
diff --git a/chain/src/item/default/splone.rs b/chain/src/item/default/splone.rs
index 09fca0b..c73bd9f 100644
--- a/chain/src/item/default/splone.rs
+++ b/chain/src/item/default/splone.rs
@@ -290,7 +290,8 @@ impl DefaultForest<ForestLabel<GrammarLabel>> {
     ///
     /// 2. Preserve the old children as specified by `edge_index + 1`.
     ///
-    /// 3. For each parent, clone the parent.  Replace the original
+    /// 3. If the new node is closed or if the new node is not cloned,
+    /// then for each parent, clone the parent.  Replace the original
     /// child of the parent, which pointed to the old node, by this
     /// new node.  Other children are inherited from the old parent.
     ///
@@ -305,15 +306,16 @@ impl DefaultForest<ForestLabel<GrammarLabel>> {
         completingp: bool,
     ) -> Result<usize, Error> {
         let end = new_label.label().end();
+        let new_node_is_clone = new_label.clone_index().is_some();
 
         let mut builder = PLGBuilderMut::from_graph_mut(&mut self.graph);
 
         let new_node = builder.add_vertex(new_label);
 
-        let new_packed = if new_label.clone_index().is_some() {
+        let new_packed = if new_node_is_clone {
             let packed = builder
                 .query_label(ForestLabel::new(new_label.label(), ForestLabelType::Packed))
-                .unwrap();
+                .unwrap_or_else(|| panic!("A cloned node {new_node} without packed nodes"));
 
             builder.add_edge(packed, new_node, new_label)?;
 
@@ -328,6 +330,24 @@ impl DefaultForest<ForestLabel<GrammarLabel>> {
             builder.add_edge(new_node, child, new_label)?;
         }
 
+        // NOTE: If the label of the parent is closed while the node
+        // label is open ended, we need to use an opened parent
+        // instead.
+        //
+        // To be specific, we only need to split the parent if the new
+        // label is closed.  This can be seen from the fact that this
+        // function only deals with the situation that the new label
+        // and the old label are different.  In this situation, if the
+        // new label is closed, it means we are closing a node under
+        // an open node, and we need to clone that parent indeed.  On
+        // the other hand, if we are opening a node under a closed
+        // node, that parent does not need to connect to this new
+        // node.
+
+        if end.is_none() && new_node_is_clone {
+            return Ok(new_node);
+        }
+
         let parents: Vec<_> = {
             if let Some(label) = self.vertex_label(node)? {
                 if label.clone_index().is_some() {