src/Pure/Isar/token.scala

     kind == Token.Kind.NAT
   def is_xname: Boolean = is_name || kind == Token.Kind.LONG_IDENT
   def is_text: Boolean = is_xname || kind == Token.Kind.VERBATIM
   def is_space: Boolean = kind == Token.Kind.SPACE
   def is_comment: Boolean = kind == Token.Kind.COMMENT
+  def is_improper: Boolean = is_space || is_comment
   def is_proper: Boolean = !is_space && !is_comment
   def is_error: Boolean = kind == Token.Kind.ERROR
   def is_unparsed: Boolean = kind == Token.Kind.UNPARSED
 
   def is_unfinished: Boolean = is_error &&