Parser: E_BACKSLASH should be two backslashes

lily/parser.yy

 /* -*- mode: c++; c-file-style: "linux"; indent-tabs-mode: t -*- */
 /*
   This file is part of LilyPond, the GNU music typesetter.
 
   Copyright (C) 1997--2015 Han-Wen Nienhuys <hanwen@xs4all.nl>
                  Jan Nieuwenhuizen <janneke@gnu.org>
 
   LilyPond is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
@@ skipping 297 matching lines @@
 
 %token CHORD_BASS "/+"
 %token CHORD_CARET "^"
 %token CHORD_COLON ":"
 %token CHORD_MINUS "-"
 %token CHORD_SLASH "/"
 %token ANGLE_OPEN "<"
 %token ANGLE_CLOSE ">"
 %token DOUBLE_ANGLE_OPEN "<<"
 %token DOUBLE_ANGLE_CLOSE ">>"
-%token E_BACKSLASH "\\"
+%token E_BACKSLASH "\\\\"
 %token E_EXCLAMATION "\\!"
 %token E_PLUS "\\+"
 %token EXTENDER "__"
 
 /*
 If we give names, Bison complains.
 */
 %token FIGURE_CLOSE /* "\\>" */
 %token FIGURE_OPEN /* "\\<" */
 %token FIGURE_SPACE "_"
@@ skipping 4400 matching lines @@
         Lily_lexer *lex = parser->lexer_;
 
         lex->lexval_ = s;
         lex->lexloc_ = loc;
         int tok = lex->pop_extra_token ();
         if (tok >= 0)
                 return tok;
         lex->prepare_for_next_token ();
         return lex->yylex ();
 }