src/Pure/General/symbol.ML

   val space: symbol
   val is_space: symbol -> bool
   val comment: symbol
   val cancel: symbol
   val latex: symbol
+  val marker: symbol
   val is_char: symbol -> bool
   val is_utf8: symbol -> bool
   val is_symbolic: symbol -> bool
   val is_symbolic_char: symbol -> bool
   val is_printable: symbol -> bool

 end;
 
 val comment = "\<comment>";
 val cancel = "\<^cancel>";
 val latex = "\<^latex>";
+val marker = "\<^marker>";
 
 fun is_char s = size s = 1;
 
 fun is_utf8 s = size s > 0 andalso forall_string (fn c => ord c >= 128) s;