src/Pure/General/symbol.ML

   val kind: symbol -> kind
   val is_letter: symbol -> bool
   val is_digit: symbol -> bool
   val is_quasi: symbol -> bool
   val is_blank: symbol -> bool
+  val is_block_ctrl: symbol -> bool
   val is_quasi_letter: symbol -> bool
   val is_letdig: symbol -> bool
   val is_ident: symbol list -> bool
   val beginning: int -> symbol list -> string
   val scanner: string -> (string list -> 'a * string list) -> symbol list -> 'a

 fun is_letter s = kind s = Letter;
 fun is_digit s = kind s = Digit;
 fun is_quasi s = kind s = Quasi;
 fun is_blank s = kind s = Blank;
 
+val is_block_ctrl = member (op =) ["\\<^bsub>", "\\<^esub>", "\\<^bsup>", "\\<^esup>"];
+
 fun is_quasi_letter s = let val k = kind s in k = Letter orelse k = Quasi end;
 fun is_letdig s = let val k = kind s in k = Letter orelse k = Digit orelse k = Quasi end;
 
 fun is_ident [] = false
   | is_ident (c :: cs) = is_letter c andalso forall is_letdig cs;