New grammar fragment for string in ixml grammar

If we agree on eliminating the difference between dstring and sstring, here 
is the proposed change to the relevant part of the grammar (the difference 
is in @string):


     -terminal: literal; 
                charset.
       literal: quoted;
                encoded.
       -quoted: (tmark, s)?, string, s


        @tmark: ["^-"].
       @string: -'"', dchar+, -'"';
                -"'", schar+, -"'".
         dchar: ~['"'; #a; #d];
                '"', -'"'. {all characters except line breaks; quotes must 
be doubled}
         schar: ~["'"; #a; #d];
                "'", -"'". {all characters except line breaks; quotes must 
be doubled}
      -encoded: (tmark, s)?, -"#", hex, s.
          @hex: ["0"-"9"; "a"-"f"; "A"-"F"]+.


Steven