diff --git a/tools/toml.scala b/tools/toml.scala
--- a/tools/toml.scala
+++ b/tools/toml.scala
@@ -1,388 +1,394 @@
 /* Author: Fabian Huch, TU Muenchen
 
 Support for TOML: https://toml.io/en/v1.0.0
  */
 package afp
 
 
 import isabelle._
 
 import scala.collection.immutable.ListMap
 import scala.reflect.{ClassTag, classTag}
 import scala.util.Try
 import scala.util.matching.Regex
 import scala.util.parsing.combinator.Parsers
 import scala.util.parsing.combinator.lexical.Scanners
 import scala.util.parsing.input.CharArrayReader.EofCh
 
 import java.time.{LocalDate, LocalDateTime, LocalTime, OffsetDateTime}
 
 
 object TOML {
   type Key = String
   type V = Any
 
   type T = Map[Key, V]
 
   object T {
     def apply(entries: (Key, V)*): T = ListMap(entries: _*)
 
     def apply(entries: List[(Key, V)]): T = ListMap(entries: _*)
 
     def unapply(t: Map[_, V]): Option[T] = {
       if (t.keys.forall(_.isInstanceOf[Key])) Some(t.asInstanceOf[T]) else None
     }
   }
 
 
   /* typed access */
 
   def split_as[A: ClassTag](map: T): List[(Key, A)] = map.keys.toList.map(k => k -> get_as[A](map, k))
 
+  def optional_as[A: ClassTag](map: T, name: String): Option[A] = map.get(name) match {
+    case Some(value: A) => Some(value)
+    case Some(value) => error("Value " + quote(value.toString) + " not of type " + classTag[A].runtimeClass.getName)
+    case None => None
+  }
+
   def get_as[A: ClassTag](map: T, name: String): A = map.get(name) match {
     case Some(value: A) => value
     case Some(value) => error("Value " + quote(value.toString) + " not of type " + classTag[A].runtimeClass.getName)
     case None => error("Field " + name + " not in " + map)
   }
 
 
   /* lexer */
 
   object Kind extends Enumeration {
     val KEYWORD, IDENT, STRING, COMMENT, ERROR = Value
   }
 
   sealed case class Token(kind: Kind.Value, text: String) {
     def is_ident: Boolean = kind == Kind.IDENT
     def is_keyword(name: String): Boolean = kind == Kind.KEYWORD && text == name
     def is_string: Boolean = kind == Kind.STRING
     def is_error: Boolean = kind == Kind.ERROR
   }
 
   object Lexer extends Scanners with Scan.Parsers {
     override type Elem = Char
     type Token = TOML.Token
 
     def errorToken(msg: String): Token = Token(Kind.ERROR, msg)
 
     val white_space: String = " \t\n\r"
     override val whiteSpace: Regex = ("[" + white_space + "]+").r
 
     override def whitespace: Parser[Any] = many(character(white_space.contains(_)))
 
     override def comment: Parser[String] =
       '#' ~>! rep(elem("", c => c != EofCh && c != '\n')) ~ elem("", c => c == EofCh || c == '\n') ^^
         { case s ~ t => s.mkString + t }
 
     def keyword: Parser[Token] = ("[[" | "]]" | one(character("{}[],=.".contains(_)))) ^^ (s => Token(Kind.KEYWORD, s))
 
     def ident: Parser[Token] =
       many1(character(c => Symbol.is_ascii_digit(c) || Symbol.is_ascii_letter(c) || c == '_' || c == '-')) ^^
         (s => Token(Kind.IDENT, s))
 
     def basic_string: Parser[Token] =
       '\"' ~> rep(string_body) <~ '\"' ^^ (cs => Token(Kind.STRING, cs.mkString))
 
     def multiline_basic_string: Parser[Token] =
       "\"\"\"" ~>!
         rep(multiline_string_body | ("\"" | "\"\"") ~ multiline_string_body) <~
         "\"\"\"" ^^
           (cs => Token(Kind.STRING, cs.mkString.stripPrefix("\n")))
 
     def multiline_string_body: Parser[Char] =
       elem("", c => c == '\t' || c == '\n' || c == '\r' || (c > '\u001f' && c != '\"' && c != '\\' && c != EofCh)) |
         '\\' ~> string_escape
 
     def string_body: Parser[Char] =
       elem("", c => c > '\u001f' && c != '\"' && c != '\\' && c != EofCh) | '\\' ~> string_escape
 
     def string_escape: Parser[Char] =
       elem("", "\"\\/".contains(_)) |
         elem("", "bfnrt".contains(_)) ^^
           { case 'b' => '\b' case 'f' => '\f' case 'n' => '\n' case 'r' => '\r' case 't' => '\t' } |
         'u' ~> repeated(character("0123456789abcdefABCDEF".contains(_)), 4, 4) ^^
           (s => Integer.parseInt(s, 16).toChar)
 
     def string_failure: Parser[Token] = '\"' ~> failure("Unterminated string")
 
     def token: Parser[Token] =
       keyword | ident | (multiline_basic_string | basic_string | string_failure) | failure("Illegal character")
   }
 
 
   /* parser */
 
   trait Parser extends Parsers {
     type Elem = Token
 
     def keys: Parser[List[Key]] = rep1sep(key, $$$("."))
 
     def string: Parser[String] = elem("string", _.is_string) ^^ (_.text)
 
     def integer: Parser[Int] = token("integer", _.is_ident, _.toInt)
 
     def float: Parser[Double] = token("float", _.is_ident, _.toDouble)
 
     def boolean: Parser[Boolean] = token("boolean", _.is_ident, _.toBoolean)
 
     def offset_date_time: Parser[OffsetDateTime] = token("offset date-time", _.is_ident, OffsetDateTime.parse)
 
     def local_date_time: Parser[LocalDateTime] = token("local date-time", _.is_ident, LocalDateTime.parse)
 
     def local_date: Parser[LocalDate] = token("local date", _.is_ident, LocalDate.parse)
 
     def local_time: Parser[LocalTime] = token("local time", _.is_ident, LocalTime.parse)
 
     def array: Parser[V] = $$$("[") ~>! rep(toml_value <~ $$$(",")) <~ $$$("]")
 
     def table: Parser[T] = $$$("[") ~>! (keys <~ $$$("]")) ~! content ^?
       { case base ~ T(map) => to_map(List(base -> map)) }
 
     def inline_table: Parser[V] = $$$("{") ~>! (content ^? to_map) <~ $$$("}")
 
     def array_of_tables: Parser[T] =
       $$$("[[") ~>! (keys <~ $$$("]]")) >> { ks =>
         repsep(content ^^ to_map, $$$("[[") ~! (keys ^? { case ks1 if ks1 == ks => }) ~ $$$("]]")) ^^
           { list => List(ks -> list) } ^? to_map
       }
 
     def toml: Parser[T] = (content ~ rep(table | array_of_tables)) ^?
       { case T(map) ~ maps => map :: maps } ^? { case Merge(map) => map }
 
 
     /* auxiliary parsers */
 
     private def $$$(name: String): Parser[Token] = elem(name, _.is_keyword(name))
 
     private def token[A](name: String, p: Token => Boolean, parser: String => A): Parser[A] =
       elem(name, p) ^^ (tok => Try { parser(tok.text) }) ^? { case util.Success(v) => v }
 
     private def key: Parser[Key] = elem("key", e => e.is_ident || e.is_string) ^^ (_.text)
 
-    private def toml_value: Parser[V] = string | integer | float | offset_date_time | local_date_time | local_date |
-      local_time | array | inline_table
+    private def toml_value: Parser[V] = string | integer | float | boolean | offset_date_time |
+      local_date_time | local_date | local_time | array | inline_table
 
     private def content: Parser[List[(List[Key], V)]] = rep((keys <~ $$$("=")) ~! toml_value ^^
       { case ks ~ v => ks -> v })
 
 
     /* extractors */
 
     private object T {
       def unapply(table: List[(List[Key], V)]): Option[T] = {
         val by_first_key = table.foldLeft(ListMap.empty[Key, List[(List[Key], V)]]) {
           case (map, (k :: ks, v)) => map.updatedWith(k) {
             case Some(value) => Some(value :+ (ks, v))
             case None => Some(List((ks, v)))
           }
           case _ => return None
         }
 
         val res = by_first_key.map {
           case (k, (Nil, v) :: Nil) => k -> v
           case (k, T(map)) => k -> map
           case _ => return None
         }
 
         Some(res)
       }
     }
     private def to_map: PartialFunction[List[(List[Key], V)], T] = { case T(map) => map }
 
     object Merge {
       private def merge(map1: T, map2: T): Option[T] = {
         val res2 = map2.map {
           case (k2, v2) =>
             map1.get(k2) match {
               case Some(v1) => (v1, v2) match {
                 case (TOML.T(t1), TOML.T(t2)) => k2 -> merge(t1, t2).getOrElse(return None)
                 case x => return None
               }
               case None => k2 -> v2
             }
         }
         Some(map1.filter { case (k, _) => !map2.contains(k) } ++ res2)
       }
 
       def unapply(maps: List[T]): Option[T] = Some(maps.fold(Map.empty)(merge(_, _).getOrElse(return None)))
     }
 
 
     /* parse */
 
     def parse(input: CharSequence): T = {
       val scanner = new Lexer.Scanner(Scan.char_reader(input))
       val result = phrase(toml)(scanner)
       (result : @unchecked) match {
         case Success(toml, _) => toml
         case NoSuccess(_, next) => error("Malformed TOML input at " + next.pos)
       }
     }
   }
 
   object Parser extends Parser
 
 
   /* standard format */
 
   def parse(s: String): T = Parser.parse(s)
 
   /* format to a subset of TOML */
 
   object Format {
     def apply(toml: T): String = {
       val result = new StringBuilder
 
       /* keys */
 
       def key(k: Key): Unit = {
         val Bare_Key = """[A-Za-z0-9_-]+""".r
         k match {
           case Bare_Key() => result ++= k
           case _ =>
             result += '"'
             result ++= k
             result += '"'
         }
       }
 
       def keys(ks: List[Key]): Unit =
         Library.separate(None, ks.reverse.map(Some(_))).foreach {
           case None => result += '.'
           case Some(k) => key(k)
         }
 
       /* string */
 
       def basic_string(s: String): Unit = {
         result += '"'
         result ++=
           s.iterator.map {
             case '\b' => "\\b"
             case '\t' => "\\t"
             case '\n' => "\\n"
             case '\f' => "\\f"
             case '\r' => "\\r"
             case '"' => "\\\""
             case '\\' => "\\\\"
             case c =>
               if (c <= '\u001f' || c == '\u007f') "\\u%04x".format(c.toInt)
               else c
           }.mkString
         result += '"'
       }
 
       def multiline_basic_string(s: String): Unit = {
         result ++= "\"\"\"\n"
         result ++=
           s.iterator.map {
             case '\b' => "\\b"
             case '\t' => "\t"
             case '\n' => "\n"
             case '\f' => "\\f"
             case '\r' => "\r"
             case '"' => "\\\""
             case '\\' => "\\\\"
             case c =>
               if (c <= '\u001f' || c == '\u007f') "\\u%04x".format(c.toInt)
               else c
           }.mkString.replace("\"\"\"", "\"\"\\\"")
         result ++= "\"\"\""
       }
 
       /* integer, boolean, offset date-time, local date-time, local date, local time */
 
       object To_String {
         def unapply(v: V): Option[String] = v match {
           case _: Int | _: Double | _: Boolean | _: OffsetDateTime |
                _: LocalDateTime | _: LocalDate | _: LocalTime => Some(v.toString)
           case _ => None
         }
       }
 
       /* inline: string, float, To_String, value array */
 
       def inline(v: V, indent: Int = 0): Unit = {
         def indentation(i: Int): Unit = for (_ <- Range(0, i)) result ++= "  "
 
         indentation(indent)
         v match {
           case s: String =>
             if (s.contains("\n") && s.length > 20) multiline_basic_string(s)
             else basic_string(s)
           case To_String(s) =>
             result ++= s
           case list: List[V] =>
             if (list.isEmpty) {
               result ++= "[]"
             } else {
               result ++= "[\n"
               list.foreach { elem =>
                 inline(elem, indent + 1)
                 result ++= ",\n"
               }
               indentation(indent)
               result += ']'
             }
           case _ => error("Not inline TOML value: " + v.toString)
         }
       }
 
       /* array */
 
       def inline_values(path: List[Key], v: V): Unit = {
         v match {
           case T(map) => map.foreach { case (k, v1) => inline_values(k :: path, v1) }
           case _ =>
             keys(path)
             result ++= " = "
             inline(v)
             result += '\n'
         }
       }
 
       def is_inline(elem: V): Boolean = elem match {
         case To_String(_) | _: Double | _: String => true
         case list: List[V] => list.forall(is_inline)
         case _ => false
       }
 
       def array(path: List[Key], list: List[V]): Unit = {
         if (list.forall(is_inline)) inline_values(path.take(1), list)
         else {
           list.collect {
             case T(map) =>
               result ++= "\n[["
               keys(path)
               result ++= "]]\n"
               table(path, map, is_table_entry = true)
             case _ => error("Array can only contain either all tables or all non-tables")
           }
         }
       }
 
       /* table */
 
       def table(path: List[Key], map: T, is_table_entry: Boolean = false): Unit = {
         val (values, nodes) = map.toList.partition(kv => is_inline(kv._2))
 
         if (!is_table_entry && path.nonEmpty) {
           result ++= "\n["
           keys(path)
           result ++= "]\n"
         }
 
         values.foreach { case (k, v) => inline_values(List(k), v) }
 
         nodes.foreach {
           case (k, T(map1)) => table(k :: path, map1)
           case (k, list: List[V]) => array(k :: path, list)
           case (_, v) => error("Invalid TOML: " + v.toString)
         }
       }
 
       table(Nil, toml)
       result.toString
     }
   }
 }