diff --git a/src/Pure/General/bytes.scala b/src/Pure/General/bytes.scala
--- a/src/Pure/General/bytes.scala
+++ b/src/Pure/General/bytes.scala
@@ -1,206 +1,212 @@
 /*  Title:      Pure/General/bytes.scala
     Author:     Makarius
 
 Immutable byte vectors versus UTF8 strings.
 */
 
 package isabelle
 
 
 import java.io.{File => JFile, ByteArrayOutputStream, ByteArrayInputStream,
   OutputStream, InputStream, FileInputStream, FileOutputStream}
 import java.net.URL
 
 import org.tukaani.xz.{XZInputStream, XZOutputStream}
 
 
 object Bytes {
   val empty: Bytes = new Bytes(Array[Byte](), 0, 0)
 
   def apply(s: CharSequence): Bytes = {
     val str = s.toString
     if (str.isEmpty) empty
     else {
       val b = UTF8.bytes(str)
       new Bytes(b, 0, b.length)
     }
   }
 
   def apply(a: Array[Byte]): Bytes = apply(a, 0, a.length)
 
   def apply(a: Array[Byte], offset: Int, length: Int): Bytes =
     if (length == 0) empty
     else {
       val b = new Array[Byte](length)
       System.arraycopy(a, offset, b, 0, length)
       new Bytes(b, 0, b.length)
     }
 
   val newline: Bytes = apply("\n")
 
 
   /* base64 */
 
   def decode_base64(s: String): Bytes = {
     val a = Base64.decode(s)
     new Bytes(a, 0, a.length)
   }
 
 
   /* read */
 
   def read_stream(stream: InputStream, limit: Int = Integer.MAX_VALUE, hint: Int = 1024): Bytes =
     if (limit == 0) empty
     else {
       val out_size = (if (limit == Integer.MAX_VALUE) hint else limit) max 1024
       val out = new ByteArrayOutputStream(out_size)
       val buf = new Array[Byte](8192)
       var m = 0
 
       while ({
         m = stream.read(buf, 0, buf.length min (limit - out.size))
         if (m != -1) out.write(buf, 0, m)
         m != -1 && limit > out.size
       }) ()
 
       new Bytes(out.toByteArray, 0, out.size)
     }
 
   def read(file: JFile): Bytes = {
     val length = file.length
     val limit = if (length < 0 || length > Integer.MAX_VALUE) Integer.MAX_VALUE else length.toInt
     using(new FileInputStream(file))(read_stream(_, limit = limit))
   }
 
   def read(path: Path): Bytes = read(path.file)
 
   def read(url: URL): Bytes = using(url.openStream)(read_stream(_))
 
 
   /* write */
 
   def write(file: JFile, bytes: Bytes): Unit =
     using(new FileOutputStream(file))(bytes.write_stream(_))
 
   def write(path: Path, bytes: Bytes): Unit = write(path.file, bytes)
 }
 
 final class Bytes private(
   protected val bytes: Array[Byte],
   protected val offset: Int,
   val length: Int) extends CharSequence {
   /* equality */
 
   override def equals(that: Any): Boolean = {
     that match {
       case other: Bytes =>
         if (this eq other) true
         else if (length != other.length) false
         else (0 until length).forall(i => bytes(offset + i) == other.bytes(other.offset + i))
       case _ => false
     }
   }
 
   private lazy val hash: Int = {
     var h = 0
     for (i <- offset until offset + length) {
       val b = bytes(i).asInstanceOf[Int] & 0xFF
       h = 31 * h + b
     }
     h
   }
 
   override def hashCode(): Int = hash
 
 
   /* content */
 
   lazy val sha1_digest: SHA1.Digest = SHA1.digest(bytes)
 
   def is_empty: Boolean = length == 0
 
   def iterator: Iterator[Byte] =
     for (i <- (offset until (offset + length)).iterator)
       yield bytes(i)
 
   def array: Array[Byte] = {
     val a = new Array[Byte](length)
     System.arraycopy(bytes, offset, a, 0, length)
     a
   }
 
   def text: String = UTF8.decode_permissive(this)
 
+  def wellformed_text: Option[String] = {
+    val s = text
+    if (this == Bytes(s)) Some(s) else None
+  }
+
   def encode_base64: String = {
     val b =
       if (offset == 0 && length == bytes.length) bytes
       else Bytes(bytes, offset, length).bytes
     Base64.encode(b)
   }
 
-  def maybe_encode_base64: (Boolean, String) = {
-    val s = text
-    if (this == Bytes(s)) (false, s) else (true, encode_base64)
-  }
+  def maybe_encode_base64: (Boolean, String) =
+    wellformed_text match {
+      case Some(s) => (false, s)
+      case None => (true, encode_base64)
+    }
 
   override def toString: String = "Bytes(" + length + ")"
 
   def proper: Option[Bytes] = if (is_empty) None else Some(this)
   def proper_text: Option[String] = if (is_empty) None else Some(text)
 
   def +(other: Bytes): Bytes =
     if (other.is_empty) this
     else if (is_empty) other
     else {
       val new_bytes = new Array[Byte](length + other.length)
       System.arraycopy(bytes, offset, new_bytes, 0, length)
       System.arraycopy(other.bytes, other.offset, new_bytes, length, other.length)
       new Bytes(new_bytes, 0, new_bytes.length)
     }
 
 
   /* CharSequence operations */
 
   def charAt(i: Int): Char =
     if (0 <= i && i < length) (bytes(offset + i).asInstanceOf[Int] & 0xFF).asInstanceOf[Char]
     else throw new IndexOutOfBoundsException
 
   def subSequence(i: Int, j: Int): Bytes = {
     if (0 <= i && i <= j && j <= length) new Bytes(bytes, offset + i, j - i)
     else throw new IndexOutOfBoundsException
   }
 
   def trim_line: Bytes =
     if (length >= 2 && charAt(length - 2) == 13 && charAt(length - 1) == 10)
       subSequence(0, length - 2)
     else if (length >= 1 && (charAt(length - 1) == 13 || charAt(length - 1) == 10))
       subSequence(0, length - 1)
     else this
 
 
   /* streams */
 
   def stream(): ByteArrayInputStream = new ByteArrayInputStream(bytes, offset, length)
 
   def write_stream(stream: OutputStream): Unit = stream.write(bytes, offset, length)
 
 
   /* XZ data compression */
 
   def uncompress(cache: XZ.Cache = XZ.Cache()): Bytes =
     using(new XZInputStream(stream(), cache))(Bytes.read_stream(_, hint = length))
 
   def compress(options: XZ.Options = XZ.options(), cache: XZ.Cache = XZ.Cache()): Bytes = {
     val result = new ByteArrayOutputStream(length)
     using(new XZOutputStream(result, options, cache))(write_stream(_))
     new Bytes(result.toByteArray, 0, result.size)
   }
 
   def maybe_compress(
     options: XZ.Options = XZ.options(),
     cache: XZ.Cache = XZ.Cache()
   ) : (Boolean, Bytes) = {
     val compressed = compress(options = options, cache = cache)
     if (compressed.length < length) (true, compressed) else (false, this)
   }
 }