renaming core to chain and some other changes

Some changes:

- The core crate is renamed to "chain".

- The crate "viz" is added, which will provide layered graph drawing
  algorithms.

- A function is added to convert from a grammar to the regular
  language of its left-linear closures.

- A function is added to convert from a nondeterministic finite
  automaton to its "null" closure.  A null closure is the same
  automaton with edges added, as if some edges are "null".  Whether an
  edge is null is determined by a function.
  Combined with the previous change, we can convert a grammar to the
  regular language of the null closure of its left-linear closures.

---

Now it remains to test more grammars and add an Atom trait, before
finishing the part about compilations.

1 files changed, 7 insertions, 2 deletions

diff --git a/graph/src/error.rs b/graph/src/error.rs
index 2162685..3600005 100644
--- a/graph/src/error.rs
+++ b/graph/src/error.rs
@@ -1,16 +1,20 @@
 #![warn(missing_docs)]
 //! This file implements the error data type of the graph library.
 
-use std::fmt::{self, Display};
+use core::fmt::{self, Display};
 
 /// The error type for methods of the trait [`Graph`][`super::Graph`].
-#[derive(Debug, Clone, PartialEq, Eq, Ord, PartialOrd)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Ord, PartialOrd)]
+#[non_exhaustive]
 pub enum Error {
     /// The index is out of bounds.
     ///
     /// The first component is the index that is out of bounds, and
     /// 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,
 }
 
 impl Display for Error {
@@ -19,6 +23,7 @@ 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"),
         }
     }
 }