before a major refactor

I decide to adopt a new approach of recording and updating item
derivation forests.  Since this affects a lot of things, I decide to
commit before the refactor, so that I can create a branch for that
refactor.

1 files changed, 20 insertions, 0 deletions

diff --git a/grammar/src/lib.rs b/grammar/src/lib.rs
index ab0f693..21ce2b4 100644
--- a/grammar/src/lib.rs
+++ b/grammar/src/lib.rs
@@ -25,6 +25,12 @@ use std::{
     fmt::Display,
 };
 
+/// The index of the starting non-terminal.
+///
+/// By convention this is the zero-th non-terminal.  I define this
+/// constant just for the sake of clarity.
+pub const START_NONTERMINAL: usize = 0;
+
 /// The type of a terminal.
 ///
 /// For the time being this is a wrapper around a string, but in the
@@ -478,8 +484,17 @@ impl Grammar {
 
     /// Return true if and only if the terminal can appear as the
     /// first terminal in a string expanded from the non-terminal.
+    ///
+    /// # Errors
+    ///
+    /// If `non_terminal` or `terminal` is out of bounds, the function
+    /// returns an error indicating this fact.
     #[inline]
     pub fn is_first_of(&self, non_terminal: usize, terminal: usize) -> Result<bool, Error> {
+        if terminal >= self.ter_num() {
+            return Err(Error::IndexOutOfBounds(terminal, self.ter_num()));
+        }
+
         Ok(self
             .firsts
             .get(non_terminal)
@@ -488,6 +503,11 @@ impl Grammar {
     }
 
     /// Return true if and only if the non-terminal is nullable.
+    ///
+    /// # Error
+    ///
+    /// If `non_terminal` is out of bounds, return the corresponding
+    /// error.
     #[inline]
     pub fn is_nullable(&self, non_terminal: usize) -> Result<bool, Error> {
         Ok(self