1 files changed, 28 insertions, 0 deletions
diff --git a/chain/src/atom/mod.rs b/chain/src/atom/mod.rs
index 398edd2..c9dadb2 100644
--- a/chain/src/atom/mod.rs
+++ b/chain/src/atom/mod.rs
@@ -17,6 +17,16 @@ pub trait Atom: Nfa<LabelType<TNT>> + Deref<Target = Grammar> {
     /// left-linear null closure of `nt` with respect to `t`.
     fn atom(&self, nt: usize, t: usize) -> Result<Option<usize>, GrammarError>;
 
+    /// A type that iterates through the rule positions.
+    type Iter<'a>: Iterator<Item = usize> + 'a
+    where
+        Self: 'a;
+
+    /// Return an iterator of rule positions responsible for an edge
+    /// from the virtual node corresponding to the non-terminal `nt`
+    /// and terminal `t` to `target`.
+    fn trace(&self, nt: usize, t: usize, target: usize) -> Option<<Self as Atom>::Iter<'_>>;
+
     /// Return the index of the empty state.
     fn empty(&self) -> usize;
 
@@ -33,6 +43,9 @@ mod tests {
     use super::*;
     use grammar::test_grammar_helper::*;
 
+    #[cfg(feature = "test-print-viz")]
+    use graph::Graph;
+
     #[test]
     fn atom() -> Result<(), Box<dyn std::error::Error>> {
         let grammar = new_notes_grammar()?;
@@ -41,6 +54,21 @@ mod tests {
 
         println!("atom = {atom}");
 
+        #[cfg(feature = "test-print-viz")]
+        {
+            println!("virtual frag for 1, 3: ");
+
+            for (index, frag) in atom
+                .generate_virtual_frags(1, 3, None)
+                .iter()
+                .flatten()
+                .enumerate()
+            {
+                crate::item::default::print_labels(&atom, *frag)?;
+                frag.print_viz(&format!("frag {index}.gv"))?;
+            }
+        }
+
         Ok(())
     }
 }