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• Is it easier to write a recursive-descent parser using an EBNF or a BNF?

  - by Vivin Paliath

  I've got a BNF and EBNF for a grammar. The BNF is obviously more verbose. I have a fairly good idea as far as using the BNF to build a recursive-descent parser; there are many resources for this. I am having trouble finding resources to convert an EBNF to a recursive-descent parser. Is this because it's more difficult? I recall from my CS theory classes that we went over EBNFs, but we didn't go over converting them into a recursive-descent parser. We did go over converting BNF's into a recursive-descent parser. The reason I'm asking is because the EBNF is more compact. From looking at the EBNF's in general, I notice that terms enclosed between { and } can be converted into a while loop. Are there any other guidelines or rules?

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• Can't find controls in FormView.InsertItemTemplate even on DataBound event

  - by abatishchev

  I have FormView in my page markup: <asp:FormView ruanat="server" ID="FormView1" OnDataBound="FormView1_DataBound" DataSourceID="SqlDataSource1"> <InsertItemTemplate> <uc:UserControl1 runat="server" ID="ucUserControl1" /> </InsertItemTemplate> </asp:FormView> <asp:SqlDataSource runat="server" ID="SqlDataSource1" SelectCommand="EXEC someSP" /> It's code-behind: protected void FormView1_DataBound (object sender, EventArgs e) { var c1 = ((FormView)sender).FindControl("ucUserControl1"); // returns null var c2 = FormView1.FindControl("ucUserControl1"); // also returns null } In theory, I'm able to find control on FormView after it being data bound. But I'm not. Why?

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• Which programming languages aren't considered high-level?

  - by hilo

  In informatics theory I hear and read about high-level and low-level languages all time. Yet I don't understand why this is still relevant as there aren't any (relevant) low-level languages except assembler in use today. So you get: Low-level Assembler Definitely not low-level C BASIC FORTRAN COBOL ... High-level C++ Ruby Python PHP ... And if assembler is low-level, how could you put for example C into the same list. I mean: C is extremely high-level compared to assembler. Same even for COBOL, Fortran, etc. So why does everybody keep mentioning high and low-level languages if assembler is really the only low-level language.

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• Studying MySQL, SQLite source code to learn about RDBMS implementation

  - by Yang

  I know implementing database is a huge topic, but I want to have a basic understanding of how database systems work (e.g. memory management, binary tree, transaction, sql parsing, multi-threading, partitions, etc) by investigating the source code of the database. Since there are a few already proven very robust open source databases like mysql, sqlite and so on. However, the code are very complicated and I have no clue where to start. Also I find that the old school database textbooks are only explaining the theory, not the implementation details. Can anyone suggest how I should get started and if there are any books that emphasis on the technology and techniques of building dbms used in modern database industry?

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• Covariance and Contravariance on the same type argument

  - by William Edmondson

  The C# spec states that an argument type cannot be both covariant and contravariant at the same time. This is apparent when creating a covariant or contravariant interface you decorate your type parameters with "out" or "in" respectively. There is not option that allows both at the same time ("outin"). Is this limitation simply a language specific constraint or are there deeper, more fundamental reasons based in category theory that would make you not want your type to be both covariant and contravariant? Edit: My understanding was that arrays were actually both covariant and contravariant. public class Pet{} public class Cat : Pet{} public class Siamese : Cat{} Cat[] cats = new Cat[10]; Pet[] pets = new Pet[10]; Siamese[] siameseCats = new Siamese[10]; //Cat array is covariant pets = cats; //Cat array is also contravariant since it accepts conversions from wider types cats = siameseCats;

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• Building an XML editor in java and RichFaces

  - by Mark Lewis

  Hello I'm building an XML editor using the above technologies. In essence, I want to read in a whole XML file to a java object, and refer using this object to each element in the XML node tree (grouped into entries) to display the content locked, have separate padlocks for the user to click to 'unlock' an entry allow overwriting of the data, and to submit this entry. 'Add entry', 'Duplicate entry', 'Delete entry' are also functions I'd like to add. I already use dom4j and XPath to access areas of the XML file so some of the work in theory is already done. Given the above, I was going to use these two together with inplaceInputs to allow the user to edit the XML and JSF validators to check the data coming in. Is this the best way to approach this problem, or is there a more straightforward route than XPathing a whole record? I started looking at jaxb but I'm new at java and jsf but I've got the feeling I won't be by the end.. Thanks

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• OpenGL Game development for the Desktop / Platform issues

  - by anon

  Does anyone have experience writing professional OpenGL games on Windows? For the Mac, due to apple's control, the OS seems quite "uniform". For windows, due to different hardware, different drivers installed, etc ... the hardware base seems to have many many different configurations. In theory, OpenGL provides a API that abstracts all this away. In practice, many drivers are often buggy and have weird cases -- has this been a problem for developing games on Windows? Thanks!

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• What is the right tool to detect VMT or heap corruption in Delphi ?

  - by Roland Bengtsson

  I'm a member in a team that use Delphi 2007 for a larger application and we suspect heap corruption because sometimes there are strange bugs that have no other explanation. I believe that the Rangechecking option for the compiler is only for arrays. I want a tool that give an exception or log when there is a write on a memory address that is not allocated by the application. Regards EDIT: The error is of type: Error: Access violation at address 00404E78 in module 'BoatLogisticsAMCAttracsServer.exe'. Read of address FFFFFFDD EDIT2: Thanks for all suggestions. Unfortunately I think that the solution is deeper than that. We use a patched version of Bold for Delphi as we own the source. Probably there are some errors introduced in the Bold framwork. Yes we have a log with callstacks that are handled by JCL and also trace messages. So a callstack with the exception can lock like this: 20091210 16:02:29 (2356) [EXCEPTION] Raised EBold: Failed to derive ServerSession.mayDropSession: Boolean OCL expression: not active and not idle and timeout and (ApplicationKernel.allinstances->first.CurrentSession <> self) Error: Access violation at address 00404E78 in module 'BoatLogisticsAMCAttracsServer.exe'. Read of address FFFFFFDD. At Location BoldSystem.TBoldMember.CalculateDerivedMemberWithExpression (BoldSystem.pas:4016) Inner Exception Raised EBold: Failed to derive ServerSession.mayDropSession: Boolean OCL expression: not active and not idle and timeout and (ApplicationKernel.allinstances->first.CurrentSession <> self) Error: Access violation at address 00404E78 in module 'BoatLogisticsAMCAttracsServer.exe'. Read of address FFFFFFDD. At Location BoldSystem.TBoldMember.CalculateDerivedMemberWithExpression (BoldSystem.pas:4016) Inner Exception Call Stack: [00] System.TObject.InheritsFrom (sys\system.pas:9237) Call Stack: [00] BoldSystem.TBoldMember.CalculateDerivedMemberWithExpression (BoldSystem.pas:4016) [01] BoldSystem.TBoldMember.DeriveMember (BoldSystem.pas:3846) [02] BoldSystem.TBoldMemberDeriver.DoDeriveAndSubscribe (BoldSystem.pas:7491) [03] BoldDeriver.TBoldAbstractDeriver.DeriveAndSubscribe (BoldDeriver.pas:180) [04] BoldDeriver.TBoldAbstractDeriver.SetDeriverState (BoldDeriver.pas:262) [05] BoldDeriver.TBoldAbstractDeriver.Derive (BoldDeriver.pas:117) [06] BoldDeriver.TBoldAbstractDeriver.EnsureCurrent (BoldDeriver.pas:196) [07] BoldSystem.TBoldMember.EnsureContentsCurrent (BoldSystem.pas:4245) [08] BoldSystem.TBoldAttribute.EnsureNotNull (BoldSystem.pas:4813) [09] BoldAttributes.TBABoolean.GetAsBoolean (BoldAttributes.pas:3069) [10] BusinessClasses.TLogonSession._GetMayDropSession (code\BusinessClasses.pas:31854) [11] DMAttracsTimers.TAttracsTimerDataModule.RemoveDanglingLogonSessions (code\DMAttracsTimers.pas:237) [12] DMAttracsTimers.TAttracsTimerDataModule.UpdateServerTimeOnTimerTrig (code\DMAttracsTimers.pas:482) [13] DMAttracsTimers.TAttracsTimerDataModule.TimerKernelWork (code\DMAttracsTimers.pas:551) [14] DMAttracsTimers.TAttracsTimerDataModule.AttracsTimerTimer (code\DMAttracsTimers.pas:600) [15] ExtCtrls.TTimer.Timer (ExtCtrls.pas:2281) [16] Classes.StdWndProc (common\Classes.pas:11583) The inner exception part is the callstack at the moment an exception is reraised. EDIT3: The theory right now is that the Virtual Memory Table (VMT) is somehow broken. When this happen there is no indication of it. Only when a method is called an exception is raised (ALWAYS on address FFFFFFDD, -35 decimal) but then it is too late. You don't know the real cause for the error. Any hint of how to catch a bug like this is really appreciated!!! We have tried with SafeMM, but the problem is that the memory consumption is too high even when the 3 GB flag is used. So now I try to give a bounty to the SO community :) EDIT4: One hint is that according the log there is often (or even always) another exception before this. It can be for example optimistic locking in the database. We have tried to raise exceptions by force but in test environment it just works fine. EDIT5: Story continues... I did a search on the logs for the last 30 days now. The result: "Read of address FFFFFFDB" 0 "Read of address FFFFFFDC" 24 "Read of address FFFFFFDD" 270 "Read of address FFFFFFDE" 22 "Read of address FFFFFFDF" 7 "Read of address FFFFFFE0" 20 "Read of address FFFFFFE1" 0 So the current theory is that an enum (there is a lots in Bold) overwrite a pointer. I got 5 hits with different address above. It could mean that the enum holds 5 values where the second one is most used. If there is an exception a rollback should occur for the database and Boldobjects should be destroyed. Maybe there is a chance that not everything is destroyed and a enum still can write to an address location. If this is true maybe it is possible to search the code by a regexpr for an enum with 5 values ? EDIT6: To summarize, no there is no solution to the problem yet. I realize that I may mislead you a bit with the callstack. Yes there are a timer in that but there are other callstacks without a timer. Sorry for that. But there are 2 common factors. An exception with Read of address FFFFFFxx. Top of callstack is System.TObject.InheritsFrom (sys\system.pas:9237) This convince me that VilleK best describe the problem. I'm also convinced that the problem is somewhere in the Bold framework. But the BIG question is, how can problems like this be solved ? It is not enough to have an Assert like VilleK suggest as the damage has already happened and the callstack is gone at that moment. So to describe my view of what may cause the error: Somewhere a pointer is assigned a bad value 1, but it can be also 0, 2, 3 etc. An object is assigned to that pointer. There is method call in the objects baseclass. This cause method TObject.InheritsForm to be called and an exception appear on address FFFFFFDD. Those 3 events can be together in the code but they may also be used much later. I think this is true for the last method call. EDIT7: We work closely with the the author of Bold Jan Norden and he recently found a bug in the OCL-evaluator in Bold framework. When this was fixed these kinds of exceptions decreased a lot but they still occasionally come. But it is a big relief that this is almost solved.

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• Algorithm and data structure learning resources for dynamic programming

  - by Pranav

  Im learning dynamic programming now, and while I know the theory well, designing DP algorithms for new problems is still difficult. This is what i would really like now- A book or a website, which poses a problem which can be solved by dynamic programming. Also there is the solution with an explanation available, which i would like to see if i cant solve the problem even after butting my head at it for a few hours. Is there some resource that provides this sort of a thing for several categories of algorithms- like graph algorithms, dynamic programming, etc? P.S. I considered Topcoder, but the solutions there are not really appropriate for learning to implement efficient solutions.

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• Implementing PageRank using MapReduce

  - by Nick D.

  Hello, I'm trying to get my head around an issue with the theory of implementing the PageRank with MapReduce. I have the following simple scenario with three nodes: A B C. The adjacency matrix is here: A { B, C } B { A } The PageRank for B for example is equal to: (1-d)/N + d ( PR(A) / C(A) ) N = number of incoming links to B PR(A) = PageRank of incoming link A C(A) = number of outgoing links from page A I am fine with all the schematics and how the mapper and reducer would work but I cannot get my head around how at the time of calculation by the reducer, C(A) would be known. How will the reducer, when calculating the PageRank of B by aggregating the incoming links to B will know the number of outgoing links from each page. Does this require a lookup in some external data source?

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• Difference between implementing an interface and applying an attribute in C#

  - by RemotecUk

  This might be a stupid question but Ill ask anyway, I was reading "OOP Demystified: A Self-Teaching Guide by Jim Keogh and Mario Giannini" chapter 11 which covers interfaces. The examples in this book are C++. I noticed that C++ uses ISerializable to make a class serializable which you would implement where as in C# you just attribute the class with the [Serializable] attribute. What is the key difference here? Is it that with an interface you must provide the implementation where as if you attribute something the compiler will work out the implementation for you? I guess that with the [Serializable] attribute the .Net framework uses reflection to make the serialized object from the actual object. That said is it possible in that case to have an [Disposable] attribute or using my theory above the framework wont know how to actually dispose of an object hence you have to do it yourself? Would be grateful for a clarification. Thanks.

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• Looking for evolutionary music example code

  - by Dan Dyer

  I would like to implement an interactive evolutionary algorithm for generating music (probably just simple melodies to start with). I'd like to use JFugue for this. Its website claims that it is well-suited to evolutionary music, but I can't find any evolutionary examples. I already have a framework to provide the evolutonary machinery. What I am looking for is some simple, working code that demonstrates viable approaches for the musical part (e.g. suitable encodings and evolutionary operators for the evolved tunes). I have some ideas how it might be achieved, but I'm not particularly knowledgeable about music theory, so to start with I'd like to just reimplement something that is known to work. So does anybody have, or know of, any freely available code (any language is fine) that demonstrates one or more approaches to evolutionary music? EDIT: I'm specifically looking for evolutionary code rather than other techniques that could be used for music synthesis.

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• iphone - testing if an object exists

  - by Mike

  I have several apps in my app that can become nil at some point and I have methods that in theory are used to put these objects to nil. But, if I try to put to nil an object that does not exist, the app will crash. for example... [object1 release]; object1 = nil; //... and after that [object removeFromSuperview]; // this will crash Then I thought, why not testing to see if the object exists before removing... if (object1 != nil) [object removeFromSuperview]; // this will crash too, because object1 cannot be tested for nil because it does not exist How can I check if the object exists before testing if it is nil? something as if (object exists( { if(object != nil)) [object removeFromSuperview) } is this possible?

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• Suggestions for Single-Page Web Application Design?

  - by DaveDev

  My view is that unless you need to change the basic structure of the user interface, you should not have to reload the page at all for any user interactions. I'd like to approach my next ASP.NET MVC project with this in mind. Can anyone suggest any principles, patterns or practices* I should consider? Excellent book, btw. Still trying to wrap my head around some of the concepts though. I thought a question like this would help link the theory to a practical design. Thanks

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• Statistical approach to chess?

  - by Chinmay Kanchi

  Reading about how Google solves the translation problem got me thinking. Would it be possible to build a strong chess engine by analysing several million games and determining the best possible move based largely (completely?) on statistics? There are several such chess databases (this is one that has 4.5 million games), and one could potentially weight moves in identical (or mirrored or reflected) positions using factors such as the ratings of the players involved, how old the game is (to factor in improvements in chess theory) etc. Any reasons why this wouldn't be a feasible approach to building a chess engine?

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• Python if statement efficiency

  - by Dennis

  A friend (fellow low skill level recreational python scripter) asked me to look over some code. I noticed that he had 7 separate statements that basically said. if ( a and b and c): do something the statements a,b,c all tested their equality or lack of to set values. As I looked at it I found that because of the nature of the tests, I could re-write the whole logic block into 2 branches that never went more than 3 deep and rarely got past the first level (making the most rare occurrence test out first). if a: if b: if c: else: if c: else: if b: if c: else: if c: To me, logically it seems like it should be faster if you are making less, simpler tests that fail faster and move on. My real questions are 1) When I say if and else, should the if be true, does the else get completely ignored? 2) In theory would if (a and b and c) take as much time as the three separate if statements would?

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• How to configure SQLite db in Visual Studio

  - by ChrisC

  I've messed with Access a little bit in the past, had one class on OO theory, and one class on console c++ apps. Now, as a hobby project, I'm undertaking to write an actual app, which will be a database app using System.Data.SQLite and C#. I have the db's table structure planned. I have System.Data.SQLite installed and connected to VS Pro. I entered my tables and columns in VS, but that's where I'm stuck. I really don't know how to finish the db set up so I can start creating queries and testing the db structure. Can someone give me guidance to online resources that will help me learn how to get the db properly set up so I can proceed with testing it? I'm hoping for online resources specific to beginners using C# and System.Data.SQLite, but I'll use the closest I can get. Thanks.

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• How to complie x64 asp.net website?

  - by Eran Betzalel

  I'm trying to compile (using Visual Studio) an ASP.Net website with the Chilkat library. The compilation fails due to this error: Could not load file or assembly 'ChilkatDotNet2, Version=9.0.8.0, Culture=neutral, PublicKeyToken=eb5fc1fc52ef09bd' or one of its dependencies. An attempt was made to load a program with an incorrect format. I've been told that this error occurs because of platform noncompliance. The weird thing is that although the compilation fails, the site works once accessed from a browser. My theory is that the IIS compilation uses csc.exe compiler from the Framework64 (64 bit) folder while the Visual Studio uses csc.exe compiler from the Framework (32 bit) folder. If this is acually it, how can I configure my Visual studio to run with the 64 bit compiler for ASP.Net sites? This is my current development configuration: Windows 7 (x64). Visual Studio 2008 Pro (x86 of course...). Chilkat library (x64) IIS/Asp.net (x64).

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• Correct permutation cycle for Verhoeff algorithm

  - by James

  Hello, I'm implementing the Verhoeff algorithm for a check digit scheme, but there seems to be some disagreement in web sources as to which permutation cycle should form the basis of the permutation table. Wikipedia uses: (36)(01589427) while apparently, Numerical Recipies uses a different cycle and this book uses: (0)(14)(23)(56789), quoted from a 1990 article by Winters. It also notes that Verhoeff used the one Wikipedia quotes. Now, my number theory is a little rusty, but the Wikipedia cycle clearly will repeat after the 8th power, while the book one will take 10, despite it saying that s^8=s. Table 2.14(b) has other errors in the 2-cycles, so this is dubious anyway. Unfortunately, I don't have copies of the original articles (and am too tight to pay/disgusted that 40-year old knowledge is still being held to ransom by publishers), nor a copy of Numerical Recipes to check (and am loath to install their paranoia-induced copy protection plug-in to view online). So does any one know which is correct? Are they both correct?

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• Different result with reverse proxy apache and lighttpd.

  - by Danny

  I have an Apache server running in reverse proxy mode in front of a Tomcat java server. It handle HTTP and HTTPS and send those request back and forth to the Tomcat server on an internal HTTP port. I'm trying to replace the reverse proxy with Lighttpd. Here's the problem: while asking for the same HTTPS url, while using Apache as the reverse proxy, the Tomcat server redirect (302) to an HTTPS page but with Lighttpd it redirect to the same page in HTTP (not HTTPS). What does Lighttpd could do different in order to have a different result from the backend server? In theory, using Apache or Lighttpd server as a reverse proxy should not change anything... but it does. Any idea? I'll try to find something by sniffing the traffic on the backend tomcat server.

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• mysql, sqlite database source code

  - by Yang

  hi guys, i know implementing database is a huge topic, but i want to have a basic understanding of how database systems works (e.g. memory management, binary tree, transaction, sql parsing, multi-threading, partitions, etc) by investigating the source code of the database, since there are a few already proven very robust open source databases like mysql, sqlite and so on. however, the code are very complicated and i have no clue where to start. also i find that the old school database textbooks are only explaining the theory, not the implementation details. Can anyone suggest how should i get started and is there any books that emphasis on the technology and techniques of building dbms used in modern database industry? Thank in advance!

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• Determining the hostname/IP address from the MX record in PHP

  - by pmmenneg

  Hi there, have a basic email domain validation script that takes a user's email domain, resolves the IP address from that and then checks that against various published blacklists. Here is how I am determining the IP: $domain = substr(strchr($email, '@'), 1); $ip = gethostbyname($domain); The problem is that some email address domains, such as [email protected], use an MX record rather than an A record, so using gethostbyname('alumni.example.net') will fail to resolve. I know when a user's email is using an MX in the email itself by using the PHP checkdnsrr function, but once at that stage am a little stuck as to how to proceed. In theory, I could parse out the 'root' domain, i.e. 'example.net' and check it, but I've not found reliable regex that can handle this task when the user could easily have an email the format of [email protected]... So, any suggestions on how to best tackle this??

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• Can I use ido-completing-read instead of completing-read everywhere?

  - by haxney

  I'm a big fan of ido-mode, so much so that I would like to use it for things like describe-function or find-tag and so on, without having to write something like in "Can I get ido-mode-style completion for searching tags in Emacs?" for each one. Both (defalias completing-read ido-completing-read) and (setf 'completing-read 'ido-completing-read) don't work, at least partly because ido-completing-read calls completing-read in its body, so any simple redefinition would result in infinite recursion. In theory, it should be possible, since the first line of the docstring for ido-completing-read is "Ido replacement for the built-in completing-read." I've looked around a bit and can't seem to find anyone else who has attempted or succeeded at it. I realize that Icicles [2] probably provides something like this, and I may end up going with that anyway, but it is a bit more of a plunge than I care to take right now. Thanks for any help.

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• Which is faster in Python: x**.5 or math.sqrt(x)?

  - by Casey

  I've been wondering this for some time. As the title say, which is faster, the actual function or simply raising to the half power? UPDATE This is not a matter of premature optimization. This is simply a question of how the underlying code actually works. What is the theory of how Python code works? I sent Guido van Rossum an email cause I really wanted to know the differences in these methods. My email: There are at least 3 ways to do a square root in Python: math.sqrt, the '**' operator and pow(x,.5). I'm just curious as to the differences in the implementation of each of these. When it comes to efficiency which is better? His response: pow and ** are equivalent; math.sqrt doesn't work for complex numbers, and links to the C sqrt() function. As to which one is faster, I have no idea...

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• Event based scheduling in excel

  - by andreas

  Hey fellas! Where i am working we have a number of contractor programmers. We handle the requirements and the project management of our products. I have been trying using various project timing and estimations techniques but cant get the hang of it yet. I have read Joel's evidence based scheduling and i was wondering if there's anything out there that can help me apply that theory? not looking for complex software but perhaps something in excel? Any help will be appreciated Andreas

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