src/Pure/General/bytes.scala

       if (offset == 0 && length == bytes.length) bytes
       else Bytes(bytes, offset, length).bytes
     Base64.getEncoder.encodeToString(b)
   }
 
+  def maybe_base64: (Boolean, String) =
+  {
+    val s = text
+    if (this == Bytes(s)) (false, s) else (true, base64)
+  }
+
   override def toString: String =
   {
     val str = text
     if (str.contains('\uFFFD')) "Bytes(" + length + ")" else str
   }