src/Pure/Isar/token.scala

     kind == Token.Kind.STRING ||
     kind == Token.Kind.ALT_STRING ||
     kind == Token.Kind.VERBATIM ||
     kind == Token.Kind.COMMENT
   def is_string: Boolean = kind == Token.Kind.STRING
+  def is_nat: Boolean = kind == Token.Kind.NAT
   def is_name: Boolean =
     kind == Token.Kind.IDENT ||
     kind == Token.Kind.SYM_IDENT ||
     kind == Token.Kind.STRING ||
     kind == Token.Kind.NAT