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, 9 insertions, 2 deletions

diff --git a/nfa/src/error.rs b/nfa/src/error.rs
index 0c6bb3c..ad75077 100644
--- a/nfa/src/error.rs
+++ b/nfa/src/error.rs
@@ -2,10 +2,11 @@
 
 use graph::error::Error as GError;
 
-use std::fmt::{Display, Formatter};
+use core::fmt::{Display, Formatter};
 
-#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd)]
 /// The error type for NFA operations.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+#[non_exhaustive]
 pub enum Error {
     /// An unknown node id is encountered.
     UnknownNode(usize),
@@ -13,8 +14,12 @@ pub enum Error {
     ///
     /// Everything is a trade-off, wink wink.
     UnsupportedOperation,
+    /// There is no root in the underlying regular expression.
+    NoRoot,
     /// This error comes from some underlying graph operation.
     Graph(GError),
+    /// A cycle is found when constructing regular expressions.
+    Cycle,
 }
 
 impl Display for Error {
@@ -23,6 +28,8 @@ impl Display for Error {
             Error::UnknownNode(id) => write!(f, "unknown node: {id}"),
             Error::UnsupportedOperation => write!(f, "unsupported operation"),
             Error::Graph(e) => write!(f, "graph error: {e}"),
+            Error::NoRoot => write!(f, "no root found"),
+            Error::Cycle => write!(f, "a cycle is found when constructing regular expressions"),
         }
     }
 }