singly labelled graphs

Now I have a new type of labelled graphs, which can index vertices by
labels, but not index edges by labels.  The biggest difference is that
I do not have to keep a hashmap of edge targets by labels, and I do
not have to guard against the duplication of nodes with the same set
of edges.  I guard against nodes with the same label, though.

Also, in this graph, both vertices and edges have one label at a time,
whereas in the previous labelled graph there can be a multitude of
edges between the same source and target nodes, but with different
labels.

Now it remains to test this type of graphs, and to think through how
we attach forest fragments to nondeterministic finite automata edges,
and how to join forest fragments together while skipping nullable
edges, in order to finish the "compilation" part.

1 files changed, 14 insertions, 3 deletions

diff --git a/graph/src/error.rs b/graph/src/error.rs
index 3600005..bf2714b 100644
--- a/graph/src/error.rs
+++ b/graph/src/error.rs
@@ -13,8 +13,11 @@ pub enum Error {
     /// the second component is the current length of nodes.
     IndexOutOfBounds(usize, usize),
     /// The graph does not permit duplicate nodes but encounters a
-    /// repeated node
-    DuplicatedNode,
+    /// repeated node.
+    DuplicatedNode(usize),
+    /// The graph does not permit duplicate edges but encounters a
+    /// repeated edge.
+    DuplicatedEdge(usize, usize),
 }
 
 impl Display for Error {
@@ -23,7 +26,15 @@ impl Display for Error {
             Error::IndexOutOfBounds(index, len) => {
                 write!(f, "index {index} out of bounds {len} ")
             }
-            Error::DuplicatedNode => write!(f, "No duplicate nodes permitted, but found one"),
+            Error::DuplicatedNode(node) => {
+                write!(f, "No duplicate nodes permitted, but found one: {node}")
+            }
+            Error::DuplicatedEdge(source, target) => {
+                write!(
+                    f,
+                    "No duplicate edges permitted, but found one from {source} to {target}"
+                )
+            }
         }
     }
 }