1 files changed, 32 insertions, 6 deletions

diff --git a/chain/src/lib.rs b/chain/src/lib.rs
index a7740c2..d7fc519 100644
--- a/chain/src/lib.rs
+++ b/chain/src/lib.rs
@@ -16,7 +16,7 @@ use graph::{error::Error as GError, LabelExtGraph};
 
 use item::default::Error as ForestError;
 
-use item::PaSe;
+use item::PaVi;
 
 /// An edge in the Chain-Rule machine.
 #[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq, Hash)]
@@ -24,18 +24,25 @@ pub struct Edge {
     /// The position in the atomic languages.
     label: usize,
     /// The source of the associated forest edge.
-    forest_source: PaSe,
+    forest_source: PaVi,
+    /// The bottom source on which we shall perform reduction.
+    ///
+    /// If this equals `forest_source`, no extra reduction needs to be
+    /// done.
+    true_source: PaVi,
     /// Whether or not this edge is "accepting".
     accepting: bool,
 }
 
 impl Edge {
     /// Construct a new edge.
-    pub fn new(label: usize, forest_source: PaSe, accepting: bool) -> Self {
+    pub fn new(label: usize, forest_source: PaVi, accepting: bool) -> Self {
+        let true_source = forest_source;
         Self {
             label,
             forest_source,
             accepting,
+            true_source,
         }
     }
 
@@ -50,9 +57,21 @@ impl Edge {
     }
 
     /// Return the associated forest edge of the edge.
-    pub fn forest_source(&self) -> PaSe {
+    pub fn forest_source(&self) -> PaVi {
         self.forest_source
     }
+
+    /// Return the associated bottom edge of the edge from which
+    /// onwards we shall perform the reduction.
+    pub fn set_true_source(&mut self, true_source: PaVi) {
+        self.true_source = true_source;
+    }
+
+    /// Return the associated bottom edge of the edge from which
+    /// onwards we shall perform the reduction.
+    pub fn true_source(&self) -> PaVi {
+        self.true_source
+    }
 }
 
 impl core::fmt::Display for Edge {
@@ -137,7 +156,7 @@ pub enum TwoLayers {
     /// the source of the associated forest edge of the second layer,
     /// and the third is a list of edges, which are the common first
     /// layers.
-    Two(usize, PaSe, bool, Vec<(Roi, usize)>),
+    Two(usize, PaVi, bool, Vec<(Roi, usize)>),
 }
 
 /// The type of a collapsed iterator.
@@ -239,12 +258,15 @@ pub trait Chain: LabelExtGraph<Edge> {
     /// An iterator that iterates all layers that need to be merged.
     type DerIter: Iterator<Item = TwoLayers>;
 
+    // FIXME: Add a parameter to control whether to manipulate the
+    // forests or not.
+
     /// Take the derivative by a terminal `t` at position `pos`.
     fn derive(&mut self, t: usize, pos: usize) -> Result<Self::DerIter, Self::Error>;
 
     /// Take the union of all derivatives.
     fn union(&mut self, der_iter: Self::DerIter) -> Result<Vec<(Roi, usize)>, Self::Error> {
-        // REVIEW: Find a way to avoid allocations.
+        // REVIEW: Think about the possibilities to avoid allocations.
         Collapsed::<_, Self>::collapse(der_iter, self)
             .collect::<Result<Vec<(Roi, usize)>, Self::Error>>()
             .map(|mut v| {
@@ -276,6 +298,10 @@ pub trait Chain: LabelExtGraph<Edge> {
 
         Ok(())
     }
+
+    /// Signal to the parser that the end of the input is reached, so
+    /// that the parser knows to generate suitable forests.
+    fn end_of_input(&mut self) -> Result<(), Self::Error>;
 }
 
 pub mod default;