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
@@ -1,114 +1,127 @@
 /*  Title:      Pure/General/bytes.scala
     Module:     PIDE
     Author:     Makarius
 
 Immutable byte vectors versus UTF8 strings.
 */
 
 package isabelle
 
 
 import java.io.{File => JFile, OutputStream, FileInputStream}
 
 
 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 = str.getBytes(UTF8.charset)
       new Bytes(b, 0, b.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)
     }
 
 
   /* read */
 
   def read(file: JFile): Bytes =
   {
     var i = 0
     var m = 0
     val n = file.length.toInt
     val bytes = new Array[Byte](n)
 
     val stream = new FileInputStream(file)
     try {
       do {
         m = stream.read(bytes, i, n - i)
         if (m != -1) i += m
       } while (m != -1 && n > i)
     }
     finally { stream.close }
 
     new Bytes(bytes, 0, bytes.length)
   }
 }
 
 final class Bytes private(
   protected val bytes: Array[Byte],
   protected val offset: Int,
-  val length: 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)
 
   override def toString: String =
     UTF8.decode_chars(s => s, bytes, offset, offset + length).toString
 
   def isEmpty: Boolean = length == 0
 
   def +(other: Bytes): Bytes =
     if (other.isEmpty) this
     else if (isEmpty) 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
+  }
+
+
   /* write */
 
   def write(stream: OutputStream): Unit = stream.write(bytes, offset, length)
 }