Added the functionality of split or clone.

I need more than the ability to clone nodes: I also need to split the
nodes.  Now this seems to be correctly added.

1 files changed, 15 insertions, 4 deletions

diff --git a/chain/src/lib.rs b/chain/src/lib.rs
index 916678f..745ad5a 100644
--- a/chain/src/lib.rs
+++ b/chain/src/lib.rs
@@ -12,9 +12,6 @@ pub mod atom;
 
 pub mod item;
 
-// TODO: We need a module for items, that serve the role of the
-// forests now.
-
 use graph::{error::Error as GError, LabelExtGraph};
 
 use item::default::Error as ForestError;
@@ -67,6 +64,13 @@ impl core::fmt::Display for Edge {
     }
 }
 
+// TODO: Define an enumeration that stands for either an edge or a
+// "phantom" edge.
+//
+// A phantom edge will be ignored when adding edges.  The phantom
+// edges will be used to determine the acceptance of a node, when and
+// only when that new node turns out to have no children.
+
 /// Each derivation is a concatenation of two items, so there are two
 /// layers.  But some items have no children and are accepting, in
 /// which case we just skip that item completely, for performance
@@ -179,7 +183,14 @@ pub trait Chain: LabelExtGraph<Edge> {
     /// Take the union of all derivatives.
     fn union(&mut self, der_iter: Self::DerIter) -> Result<Vec<(Edge, usize)>, Self::Error> {
         // REVIEW: Find a way to avoid allocations.
-        Collapsed::<_, Self>::collapse(der_iter, self).collect()
+        Collapsed::<_, Self>::collapse(der_iter, self)
+            .collect::<Result<Vec<(Edge, usize)>, Self::Error>>()
+            .map(|mut v| {
+                v.retain(|(_, target)| {
+                    *target == 0 || matches!(self.degree(*target), Ok(deg) if deg != 0)
+                });
+                v
+            })
     }
 
     /// Use chain rule to compute the derivative with respect to a