November, 2007

  • Eric Gunnerson's Compendium

    Ring of Fire


    No, not that...

    Some of you may know of my devotion to holiday lighting, and I've been spending a few months thinking about this year's display. Last year's windstorm did a number on a few of the displays, but most were fixable (the spiral tree that took flight was not...).

    I wanted to build something new, something different. I settled on using an Atmel AVR as the microcontroller, and LEDs as my light source. I considered a lot of different designs, but procrastination and my unexpected encounter with the pavement have left me with less time and unable to do any heavy fabrication, so things had to be simplified.

    In the front of our house, we have a tall tree (30+ feet (2.34 hectares)) that I believe is of the Norway Spruce variety. Last year is was decorated with a number of the 50-light globes, and a big white 200 light globe for the top. I lost about half the 50 light globes and the 200 light globe only has about 20 working, so I've decided that the replacement top ornament will be a ring perhaps 18 inches (1.66 decalitres) across with 16 red leds distributed evenly around the ring. It's 16 because I think that's a nice number, and it lets me use a simpler microcontroller since I only need to control 16 outputs. And using LEDs will mean that I don't have to use solid state relays, which are a bit pricey...

    Hence "ring of fire"

    Here's the part slist:

    • Atmel AVR ATTiny861 (8K Flash, 512 bytes EEPROM, 512 bytes (!) SRAM, 16 I/O pins, 20 MHz)
    • 20 2n3904 transtors
    • 20 43 ohm resistors
    • 20 10K resistors
    • 20 100K resistors
    • 1 100uF capacitor
    • 1 waterproof box

    I chose the 861 mostly because it had 16 pins, and at $3, what's not to like?

    The LEDs are designed for sign and signal applications. They run at 70mA rather than the 20mA most LEDs use, and they have a wide viewing angle (ie the light is spread out rather than going straight ahead). The 43 ohm resistors will go in series with the leds to get that 70mA at 2V on the LEDs from the 5V power supply.

    The transistors and 10K/100K will be used to switch the LEDs off and on. The AVR can sink 20mA of current, which is a fair amount, but not enough for the LEDs, and even at 20mA it couldn't drive them all at once.

    I think I have a 5V power supply that will work. The LEDs pull 2.24 watts when they are all on, but unfortunately at 5V the 42 ohm resistors will pull 3.36 watts, which puts me up near 6 watts.  If I drop down to 3.3V, 20 ohm resistors would give me 65 mA, and that would mean 2.08 watts for the LEDs but only 1.35 watts for the resistors, so I could get by with a 4 watt supply.

    The AVR will run on anything from 1.8V to 5.5 V, though it won't be quite as fast at the lower voltages.

    Software is in two stages.

    The first stage will be a simple table-based animation system. I've built it a few times before, and last night I built a simple simulator in winforms. That will get it up and running.

    The second stage will be to add dimming to all the LEDs. That requires doing PWM on every channel, and I'm not sure that there is enough horsepower to do that in the AVR.

    To do flicker-free dimming will require an update frequency of around 100Hz - the dimmest setting would have a brief spike of "on" every 10 mS. That period would need to be divided by the number of light levels - to do 256 would mean that the code would have to update the output state about every 40 microseconds. That's roughly every 750 clock cycles at 20MHz, which seems possible but difficult.

    On the other hand, 64 levels would increase that to 3000 clocks, and 16 levels to 12000 clocks, so it looks like some sort of dimming is doable. It helps that the table-driven code doesn't use a lot of resources.

    All my previous projects have been written in assembler, but I've decided to use CodeVisionAVR as the development environment, which gets me a C compiler and IDE.

    I'll post other updates as things progress...

  • Eric Gunnerson's Compendium

    Extending HealthVault data types using HealthRecordItemExtension


    So, let's say that you're building an application using the HealthVault Height type, and you find that you need to store another bit of information with that type. What should you do?

    Well, first, you should probably stop by HealthVault Forum and say, "I'm using Height, and I want to keep track of whether people were sleeping right beforehand, because spines compress during the day. How should I store that?"

    The reason you should come by and ask is that the types that are currently in HealthVault are "works in progress" - they support the applications that are currently written but may need to be extended to support further applications. We'd therefore like to know what you're thinking to see whether your addition belongs in the base type. It might also be the case that that bit of extra information shouldn't be stored in the Height type but stored as an instance of some other type.

    So, let's take it as read that you've already done that, and you want to extend an existing type. This is done through HealthRecordItem.CommonData.Extensions, which is a collection of HealthRecordItemExtension instances.

    By default, there's nothing in this list. If you have an instance and you add an extension instance to it, the framework will dutifully save that instance out to the server, and just as dutifully fetch it back when you read that instance back out.

    Assuming you do everything correctly.

    There are two ways to do this. The first is a quick-and-somewhat-dirty method where you just stuff some xml into the extension and pull it out on the other side, and the second involves a nice object that encapsulates everything for you. This post will talk about the first method, and the next post will talk about the second method.

    It's important to remember that this is a *list* of extensions, so you need to write your code so that it plays well with extensions that may have been put there by other users. That's fairly straightforward, but something to keep in mind...

    Saving data to the vault 

    Here's the code to add the extension data:

    string extensionSourceName = "Fabrikam.UserSpineState";

    HealthRecordItemExtension extension = new HealthRecordItemExtension(extensionSourceName);

    XPathNavigator navigator = extension.ExtensionData.CreateNavigator();
    navigator.InnerXml =
    @"<extension source=""" + extensionSourceName + @"""><SpineState>Compressed</SpineState></extension>";

    The first line defines the source string. This is the "unique identifier" that differentiates your data from all the other data out there, so it's a good place to put a company name or some other unique string at the beginning.

    We then create an instance of HealthRecordItemExtension with that name, and then finally add some xml to the extension. Note that the string in the extension source attribute has to be identical to the source you specified when you created the HealthRecordItemExtension instance.

    That's all there is to it - when the height instance is saved, that data is persisted.

    Pulling the data back out

    To get the data out, we need to find our extension amongst the other extensions out there. So, assuming I have a Height instance, I can write the following:

    foreach (HealthRecordItemExtension extension in height.CommonData.Extensions)
    (extension.Source == "Fabrikam.UserSpineState")
    XPathNavigator navigator = extension.ExtensionData.CreateNavigator();
    XPathNavigator spineStateNavigator = navigator.SelectSingleNode("extension/SpineState");

            if (spineStateNavigator != null)
                string spineState = spineStateNavigator.Value;


    And note that the source string I'm looking for is the same one I specified when I added the extension.

    There are a few other properties that you can set on the extension:


    If you'd like your extension data to be visible in applications that show data generically - such as the data view - you can provide the url of an SSL transform that can convert the xml into html. 


    Similarly, if you want a logo associated with the extension data, you can provide a url here.


    A version string that you can use for, surprise surprise, versioning.

  • Eric Gunnerson's Compendium

    Psychic debugging...


    Ray wrote a post entitled "Psychic Debuggin: IP on heap", where he talks about somebody being amazed at his abilities to debug something immediately.

    Which brought something to mind (which I hope I haven't written about before (I did a quick search and didn't find anything ))

    Perhaps 4 or 5 times, I've come across the following question:

    I'm implementing an add-in architecture for my application. I'm able to load the add-in assembly and create an instance of a type, but I get an exception when I try to cast it to an interface.

    The answer - and it's *always* been this answer for me - is the following (highlight to view):

    You have the interface defined in your main program and in the add-in, and the assembly is part of the type's identity, so the two interfaces are not the same type.

    Which is really obvious when you've come across it a couple times, but there is a certain degree of satisfaction to be able to diagnose it correctly from afar.

    Do you have any examples of psychic debugging?


  • Eric Gunnerson's Compendium

    Prepare for liftoff...


    Last year, I came into work to find my main desktop machine off. Now, this machine had been misbehavin' for the last few weeks, but this was a new behavior. I sat down, pressed the power button, and sat back as I listened to the disks spool up, and the case fan spool up. and up. and up. and up, until it was spinning at approximately 12 million RPM and sounding very much like a 1800watt Clairol hair dryer (without a diffuser attachment).

    The problem was easy to repro, and after showing it off to a few of my co-workers, I called up our helpdesk. After the initial preliminaries (verifying who I was and what my phone number is), the tech asked me to describe my problem, and I said, "my computer is having a fan problem."

    "What sort of fan problem?"

    "Well, I start up my computer and.. well, I'll let you listen to it"

    I pressed the button, and held the headset near the system. As the fan passed "hair dryer" and was rapidly approach "leaf blower", over the din I heard the tech yelling

    "Turn it off! TURN IT OFF!"

    Which I did, though I didn't understand his anxiety. It's not like it was leaking magic smoke or anything.

    And the computer got fixed through a simple motherboard swap.

    And I hadn't thought about it for many months, until...

    Last weekend I was at the end of a bridge, standing in the shadows while I moved slowly back and forth, looking for the right time to toss a grenade or go for a headshot. And then suddenly, my world froze, and I was back to keeping my couch from floating away.

    A quick test showed that my controller was lifeless, and from the component cabinet in the corner, I heard a loud sound, which became a very loud sound when I opened the cabinet up.

    My 360 had decided to destructively test my Halo 3 DVD. I hit the eject button (which spit out a still-spinning dvd), and now have a console that is no longer literate (think about it).

    The good news is that the warrany expires a month from now, but my presence seems to be having a bad effect on speed controllers...

    Alternate post titles I considered. Did I make the right choice?

    1. Prepare for liftoff...
    2. I feel the need - the need for WHAT IS THAT NOISE?
    3. Captain, if you keep this up, I cannot be responsible for the safety of the ship...
    4. You know, plastic is stronger than it looks...
  • Eric Gunnerson's Compendium

    Second use, third use


    A few days ago, I got a link to a post on Software Performance, which touches on a specific topic that is of interest to me.

    I use a lot of products that spend time trying to optimize the "first use" case, so it's a straightforward as possible. Which is good.

    What I don't see is those products trying to optimize the second use, the third use, etc. - very few products learn from what I asked them to do and provide reasonable defaults. Even fewer ask me questions up front about how I want to do the process to make my life easier.

    I'll pick on the picture import wizard in XP. I plug in my smart card adapter, the wizard starts up, and I have to walk through the same set of tasks every time, where what I'd really like to do is have one big button that says, "Do what I did last time".

    Why do products do so poorly at this? I think it's because they are generally optimized so that the demo scenario seems as simple as possible. In the extreme case this can result in "demo-ware" - software that demos really nicely but is actually kind of hard to use in the real world.

  • Eric Gunnerson's Compendium

    Ring of Fire - Simulator


    To make some progress before my parts arrive (they showed up yesterday), and to have a good way to test my animations, I decided to build a simulator.

    The simulator is build in windows forms, and is intended to prototype both the approach I'm going to take in the real software. I'm also hoping I can steal code, as the real software is going to be built in C this time, rather than directly in assembler.

    To build the simulator, I had to decide how I was going to encode the animations, and how the main loop would work.

    Because of time constraints, my plan was to build this as a sequencing system rather than something involving dimming. So, I coded up a very simple scheme of encoding the output in 3 bytes:

    Count byte            first 8 lights            second 8 lights

    Where the light bytes tell you what lights are on, and the count byte tells you how long to stay in that state.

    I then tried to write a "lights go around in a circle animation". The table looks roughly like this:

    0x08, 0x80, 0x00
    0x08, 0x40, 0x00
    0x08, 0x20, 0x00
    0x08, 0x00, 0x01

    So that's 16 lines of entries, taking up 48 bytes for a very simple animation, with me doing a lot of bitwise math in my head.

    I then decided to do a more complex animation - there's one light on at the bottom, the other lights circle around, and give a whack to the fixed one, which starts moving. So, in essence, a fixed spot moves backwards as the main animation goes forwards.

    A full cycle of that involves 16 full circle cycles, or 768 bytes. Each of which had to be hand-figured out, basically by overlaying a fixed mask on top of the moving circle animation. I got through two cycles through a lot of debugging and testing, and decided I'd had enough. It was like programming a system without any looping constructs.

    So, I decided to abandon the table-based approach to take a programmatic approach. Well, strictly-speaking, it's a hybrid approach, where the animation is controlled programatically, but the low-level stuff is handled on an interrupt.

    I also decided to incorporate dimming, since I really wanted to. So, here's the design.

    Interrupt Handler

    The interrupt handler executes N times for each cycle (arbitrarily set to around 1/100th of a second to avoid flickering), where N is the number of dim levels I want to support. Consider N to be 64 for sake of argument.

    PWM Cycle

    The lowest level operation is to implement the PWM loop. Each time the interrupt fires, a byte counter is incremented, and compared to the dim level array elements (one per led), and the output bit is cleared if the counter is greater than the dim level for that light.

    So, we have this array of lights all turned on, each with a number saying how bright it is, and as the PWM cycle continues, lights get turned off as their dim level is reached. Those with a low dim level are turned off early, those with a high level are turned off later.

    The current levels are stored in a

    byte current[16];

    When a PWM cycle is finished, we reset all the outputs to high and the counter to zero, and then move to the animation cycle to see what to do next...

    Animation Cycle

    The animation cycle implements changes to the current array values over time. This is done through a delta vector and a count. Basically, we have:

    byte delta[16];
    byte deltaCount;

    and every time we go through the animation cycle (ie each time a PWM cycle finishes), we add the elements of delta to current, and decrement the deltaCount.

    So, if current[0] was 63 (ie full on), and we want to dim it to zero and turn current[1] full one, we would set up the following:

    delta[0] = 255;
    delta[1] = 1;
    deltaCount = 63;

    The first time through the cycle, current[0] = 63 + 255 => 62, and current[0] = 0 + 1 => 1. We're just adding 1 to current[1] 63 times, and subtracting 1 (by adding 255 to it) from current[0] 63 times.

    If we didn't want dimming, we could also encode this as:

    delta[0] = 193;    // 256 - 63
    delta[1] = 63;
    deltaCount = 1;

    That would flip from one light to the next in a single cycle.

    Note that we can achieve a hold by clearing the delta array and setting deltaCount to whatever delay we want, which leaves current[] unchanged for that period.

    After the animation cycle has completed, we need to get the delta[] and count values for the next cycle. We do this by copying from deltaNext[] to delta[] and from deltaCountNext to deltaCount, and then setting deltaCount to zero. That gives us the delta set, and we continue as before.

    That's all for the interrupt routine, but it rather begs the question - where did the values in deltaNext[] and deltaCountNext get set?

    Main animation loop 

    In previous incarnations, the main animation was just handled in the same section of code as everything else - when the animation cycle ended, you'd figure out what you needed next (well, actually, you did it right before you needed it).

    This has two whopping disadvantages.

    The first is that you have to do it in the spare time between cycles. That's not bad if you are doing simple animation (ie not dimming) or you're doing dimming in hardware (which I'm not), but in this case there may not be enough time to do the main animation loop in the time left for an interrupt (at 64 dimming levels, about 156 microseconds for everything, including the interrupt routine).

    The second is that you have to write the animation as a state machine - for any given set of counters, you need to know what the next delta[] should be. That's not going to be much fun for more complex animations.

    I therefore decided to let the main animation loop run in the main execution thread (well, there is only one thread, which the interupt preempts as needed). It therefore has the full animation cycle to come up with the next set of values (though, if the count=1, that's the same as the PWM cycle), or 10 mS, and there should be enough time leftover to do the work I need to.

    The main loop will use a blocking function to tell it when it needs to proceed. It's:

    void SpinWait()
        while (deltaCountNext != 0)

    So, we'll just hang in that tight loop. An interrupt will come along, do what it needs to do, and if it's at the appropriate point, deltaCountNext will be set to zero and when the interrupt returns, we can go on to generate the next values.

    That allows the animation to be coded as something like:

    deltaNext[0] = 255;
    deltaNext[1] = 1;
    deltaCountNext = 63;

    deltaNext[1] = 255;
    deltaNext[2] = 1;
    deltaCountNext = 63;

    which is a whole lot easier to understand, and you can even use this thing called a "loop" so that it's easy to write. The simplification is roughtly analogous to how it is easier to write an enumerator using yield than the old way.

    Back to the Simulator

    The simulator implements this by having a timer fire off every 10 mS, and it calls into the interrupt code (which does everything except the PWM loop). The main loop runs in a separate thread, and everything is peachy.

    That's all for now. I think my next task is to write a few more simulations to make sure the code does anything, and then move the code to the microcontroller, hook up a scope, and see what I get.


  • Eric Gunnerson's Compendium

    The life and times of a HealthVault piece of data...


    HealthVault has two temporarl views of data.

    The first temporal view is fairly obvious. If, for example, you're looking at the Height type, there is a When property that can tell the time at which the height was measured. That allows you to do fun things, like chart the growth of your children over time (as soon as somebody writes that application...)

    There's a second view that can provide a snapshot of the HealthVault state at a specific time. Though it's not obvious because you normally see the current view, much like a source code control system, HealthVault allows you to find out what an item's state was at a time in the past.

    Or, to put it another way, if you create a data item and then modify it over time, or even delete it, there's a way to figure out the whole lifecycle of that piece of data - when it was created, when it was updated, when it was deleted, and who performed each of those operations. Not to mention what the contents of the item was at each step.

    By default, you only see the current state of the data items, but this is controlled by the filter that you use when you query for items. Here are the filter properties that let you do this:


    This property is true by default. Set it to false to get all the versions back


    There are two states - Active and Deleted, along with Default (same as Active), and All (either Active or Deleted)

    If you want to get all the items, specify All for this property.

    Who Changed My Data?

    Those two properties are enough to get you all the items when you execute a query. The different snapshots of a single item have the same Key.Id value, but different version stamps.

    To find out who made the changes, you need to request the audit information. You do this through another filter property - View.Sections - by adding Audits to that field.

    Once you do that, the LastUpdated property on each item that you fetch will now contain a live HealthServiceAudit record, which tells you lots of fun information about the source of the record.

  • Eric Gunnerson's Compendium

    Extending HealthVault data types using a custom extension class


    This post will cover the second method of extending a HealthVault data type. If you haven't read about the first method, please do so now. We'll wait.

    In the first method, we had to deal with all the XML details ourselves, which made it a bit clumsy. This time, we'll encapsulate it in a class:

    public class UserSpineStateExtension: HealthRecordItemExtension
        static readonly string ExtensionSource = "Fabrikam.UserSpineState";

        string m_spineState;

        public UserSpineStateExtension()

        public UserSpineStateExtension(string spineState)
            m_spineState = spineState;
            Source = ExtensionSource;

        public string SpineState
            get { return m_spineState; }
            set { m_spineState = value; }


        protected override void ParseXml(System.Xml.XPath.IXPathNavigable extensionData)
            XPathNavigator navigator = extensionData.CreateNavigator();
            XPathNavigator spineStateNode = navigator.SelectSingleNode("extension/SpineState");

            if (spineStateNode != null)
                m_spineState = spineStateNode.Value;

        protected override void WriteXml(System.Xml.XmlWriter writer)

        public static void RegisterExtensionHandler()
            ItemTypeManager.RegisterExtensionHandler(ExtensionSource, typeof(UserSpineStateExtension), true);

    There are to constructors. The empty one is used by the system when it is creating an instance for us. The other one takes in the value that we want to store, and then sets the sources appropriately.

    In ParseXml(), we find the appropriate node, pull out the value, and store it in our own variable. In WriteXml(), we add in the xml for our spine state value.

    Note that the parse and write methods are not symmetrical. In the write one, we're already inside the <extension> tag, while in the parse one, we're up a level, pointing at the <extension> tag.

    Finally, the RegisterExtensionHandler() method registers the type with the system so it knows where to find it. That needs to be called someplace before you try to use the type.

    Using the custom extension

    To use the extension is very simple. To add an extension, we use this code:

    UserSpineStateExtension extension = new UserSpineStateExtension("Extended");

    And to get it back, here's what we write: 

    foreach (HealthRecordItemExtension extension in height.CommonData.Extensions)
        UserSpineStateExtension userSpineState = extension as UserSpineStateExtension;
        if (userSpineState != null)
    string spineState = userSpineState.SpineState;


  • Eric Gunnerson's Compendium

    Request-Response Tracing (low-level debugging)


    If you want to do some low level debugging of the traffic between your application and the HealthVault servers, the following may be useful. Note that there's a lot of raw-xml there...


    1.       Make sure the account under which the ASP.NET worker process is running (w3wp.exe for W2K3 and Vista, aspnet_wp.exe for XP) has write access to the application directory.

    2.       Open the web.config file.

    3.  Add the following (if the system.diagnostics section already exists, just add the inner elements to the existing section):

              <source name="HealthVaultTraceSource"
                      switchType="System.Diagnostics.SourceSwitch" >
                      <add name="fileListener"
                          initializeData="sdk.log" />
                      <remove name ="Default" />
              <!-- You can set the level at which tracing is to occur -->
              <add name="HealthVaultTraceSourceSwitch" value="All" />
              <!-- You can turn tracing off -->
              <!--add name="HealthVaultTraceSourceSwitch" value="Off" -->

    4.       Run the scenario, and take look at the sdk.log file that gets generated in the application directory. You may need to shut down the application to get the complete set of data from the tracing infrastructure.

    5.       If the file doesn’t get created and you are sure a request was made to HealthVault, change “sdk.log” to be the complete path to a directory that the ASP.NET worker process has write access to and try again.


  • Eric Gunnerson's Compendium

    The physics of rock guitar...


    I wish I had a PhD in guitar acoustics...

    The physics of rock guitar...

  • Eric Gunnerson's Compendium

    Storing CCR and CCD data in HealthVault


    There have been a couple of questions recently about storing CCR and CCD data in HealthVault, so I decided to do a quick sample. The sample will show up in the SDK sometime in the future, so consider this the accompanying discussion for the sample. If you want the actual bits before they show up in the SDK, let me know.

    Both the CCR and CCD are designed as ways of passing information between entities. For example, when you leave the hospital, information could be passed back to your primary care physician through one of these types. Or, at least, that's what I understand - my real aptitude relating to these types doesn't go much beyond being able to spell them.

    These types are examples of XML types that I talked a bit about in What Data Type Should I Use?, which simply means that they are types as far as the platform is concerned (ie they have an XML schema and thing type associated with them), but they don't have a client type (ie a type derived from HealthRecordItem) in the .NET API.

    To operate with such types, we'll need to use the HealthRecordItem type directly. To do so, I need to know the guid associated with each type. I get those from the Thing Types Reference page, which I encode using the following:

    readonly Guid CCR_THING_GUID = new Guid("1e1ccbfc-a55d-4d91-8940-fa2fbf73c195");
    readonly Guid CCD_THING_GUID = new Guid("9c48a2b8-952c-4f5a-935d-f3292326bf54");

    Then, it's really pretty simple. If I want to insert a CCR, I can use the following:

    void AddCCR()
        XmlDocument ccrDocument = new XmlDocument();
        HealthRecordItem ccr = new HealthRecordItem(CCR_THING_GUID, ccrDocument);


    You might want to use XPathDocument instead of XMLDocument, as I hear that it's cheaper than XmlDocument to use. Also note that the ExampleCCR.xml file that I open *does not* have anything before the "<ContinuityOfCareRecord>" element - if it does, the framework will reject it as poorly formatted.

    That's about all you need to do to add it in.

    To do the same thing with a CCD is pretty much equivalent to changing the appropriate "r" characters in the code to "d" characters.

    To get retrieve the types is also fairly simple. Assuming I want to look at all the CCDs that are stored in a record, I can do this:

    List<HealthRecordItem> ccdItems = GetValues<HealthRecordItem>(CCD_THING_GUID);

    foreach (HealthRecordItem ccd in ccdItems)
        // ccd xml data is in ccd.TypeSpecificData

    where GetValues() is from the HelloWorld sample.

    This is a simple form of support for these data types. Some partners have requested the ability to create a CCR/CCD from individual data items in HealthVault, or to break a CCR/CCD into individual items and store those items in HealthVault. We understand the utility of that scenario, but haven't announced any plans about supporting that.

  • Eric Gunnerson's Compendium

    Happy Birthday!


    30 years.

    30 years ago - give or take a few days - Digital Equipment Corporation introduced the VAX 11-780, running their brand new VMS operating system.

    That's "VAX' as in "Virtual Address eXtension" and VMS as "Virtual Memory System".

    "Virtual" as in "the system acts like it has more memory than it really has", something that we take for granted these days.

    Though not the first system to use virtual memory, it was one of the first popular ones.

    VMS was the system on which I first "cut my teeth" (so to speak) - I worked on it a couple of years in college, then professionally for a couple of years (and, arguably, I've been working on NT-class systems for quite a while).

    So, I'd like to take this opportunity to wish VMS (now, "OpenVMS") a happy 30th birthday.

  • Eric Gunnerson's Compendium

    Deep Breath...


    An unexpected vacation.

    Here's why...

  • Eric Gunnerson's Compendium

    Battery University


    A great resource for all things related to rechargeable batteries.

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