 extension Lexer {
+  /// Classify a 'special character escape'.
+  ///
+  /// If in a custom character class:
+  /// SpecialCharEscape -> '\t' | '\r' | '\b' | '\f' | '\a' | '\e'
+  ///
+  /// Otherwise
+  /// SpecialCharEscape -> '\t' | '\r' | '\f' | '\a' | '\e'
   private static func classifyAsSpecialCharEscape(
     _ t: Character, inCustomCharClass: Bool
   ) -> Token.SpecialCharacterEscape? {

      return nil
    }
  }
  private static func classifyAsAnchor(
    _ t: Character, fromEscape escaped: Bool, inCustomCharClass: Bool
  ) -> Anchor? {

    return mc
  }

  /// Classify a given terminal character.
  ///
  /// Terminal -> Anchor | MetaChar | SpecialCharEscape | Character
  ///
  /// If .ignoreWhitespace:
  /// ' ' -> Trivia
  ///
  static func classifyTerminal(
    _ t: Character,
    fromEscape escaped: Bool,
