1 files changed, 14 insertions, 6 deletions
diff --git a/chain/src/lib.rs b/chain/src/lib.rs
index d7fc519..9de1df7 100644
--- a/chain/src/lib.rs
+++ b/chain/src/lib.rs
@@ -258,11 +258,9 @@ 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>;
+    fn derive(&mut self, t: usize, pos: usize, no_item: bool)
+        -> 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> {
@@ -279,8 +277,18 @@ pub trait Chain: LabelExtGraph<Edge> {
 
     /// Use chain rule to compute the derivative with respect to a
     /// terminal.
-    fn chain(&mut self, t: usize, pos: usize) -> Result<(), Self::Error> {
-        let der_iter = self.derive(t, pos)?;
+    ///
+    /// # Arguments
+    ///
+    /// The argument `t` is the terminal we computet the derivative
+    /// with.
+    ///
+    /// The argument `pos` is the position within the input.
+    ///
+    /// The argument `no_item` determines whether we want the item
+    /// derivation forest as well.
+    fn chain(&mut self, t: usize, pos: usize, no_item: bool) -> Result<(), Self::Error> {
+        let der_iter = self.derive(t, pos, no_item)?;
 
         let edges = self.union(der_iter)?;