Add comment to scan()

Begin the comment for scan()

parser/src/earley/parser.rs

@@ -1100,7 +1100,17 @@ impl ParserState {
         }
         true
     }
-
+
+    // scan() implements the version of Earley described in the
+    // slides for "Parsing: Earley parsing", Winter 2017/2018,
+    // Laura Kallmeyer, Heinrich Heine Universitaet, Dusseldorf,
+    // https://user.phil-fak.uni-duesseldorf.de/~kallmeyer/Parsing/earley.pdf
+    // (Retrieved 18 Sep 2024).
+
+    // An important difference between the algorithm implemented here
+    // and Kallmeyer's is that in scan(), the token scan is performed
+    // first, while in Kallmeyer it is performed last.
+
     // lexeme body only used for captures (in definitive mode)
     // and debugging (lexeme.idx used always)
     fn scan(&mut self, lexeme: &Lexeme) -> bool {