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<p class="MsoNormal"><span style="font-size:12.0pt">Alan (adding Shayan and ghc-devs)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">OK. It’s hard to keep this straight in email. Take a look at<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:5.25pt"><span style="font-size:12.0pt">https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/Instances<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Please edit and improve it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Alan & Kim Zimmerman [mailto:alan.zimm@gmail.com]
<br>
<b>Sent:</b> 13 November 2017 13:30<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Subject:</b> Re: Trees that Grow and constraints<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:12.0pt;margin-left:0cm">
At the moment, in GHC master, we have<o:p></o:p></p>
<pre>data HsOverLit p<o:p></o:p></pre>
<pre> = OverLit {<o:p></o:p></pre>
<pre> ol_ext :: (XOverLit p),<o:p></o:p></pre>
<pre> ol_val :: OverLitVal,<o:p></o:p></pre>
<pre> ol_witness :: HsExpr p} -- Note [Overloaded literal witnesses]<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre> | XOverLit<o:p></o:p></pre>
<pre> (XXOverLit p)<o:p></o:p></pre>
<pre>deriving instance (DataIdLR p p) => Data (HsOverLit p)<o:p></o:p></pre>
<pre style="margin-bottom:12.0pt"><br><br>And in HsExtension.hs we have an ever-growing constraint type defining DataIdLR<o:p></o:p></pre>
<pre style="margin-bottom:12.0pt">I am trying to remove the need for that.<o:p></o:p></pre>
<pre style="margin-bottom:12.0pt">In the Experiment.hs file, I found that using<br><br>data Experiment p<br> = Experiment {<br> e_ext :: (XEOverLit p),<br> e_val :: Int }<br> | XExperiment (XXOverLit p)<br>deriving instance Data (GhcPass 'Parsed ) => Data (Experiment (GhcPass 'Parsed ))<br>deriving instance Data (GhcPass 'Renamed ) => Data (Experiment (GhcPass 'Renamed ))<br>deriving instance Data (GhcPass 'Typechecked) => Data (Experiment (GhcPass 'Typechecked))<o:p></o:p></pre>
<pre>will compile using GHC 8.2.1, but not with GHC 8.0.2<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>Alan<o:p></o:p></pre>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On 13 November 2017 at 15:13, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt;margin-left:36.0pt">
And it looks like it could work, when bootstrapping from GHC 8.2.1, but this is still a long time away.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">I’m sorry, I still don’t know what “it” is that “could work”. Can you be more precise. I think you must be suggesting some alternative to my code
below?</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Simon</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Alan & Kim Zimmerman [mailto:<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>]
<br>
<b>Sent:</b> 13 November 2017 13:09</span><o:p></o:p></p>
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<p class="MsoNormal"><br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>><br>
<b>Subject:</b> Re: Trees that Grow and constraints<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">Yes.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">If we can solve this, it means we can get rid of the DataId and ForallXXX constraints defined in hsSyn/HsExtension.hs<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">And also move the type family definitions to the files where they are used.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">I suspect that typechecking those constraint sets is adding to the GHC compilation time, significantly<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">And it looks like it could work, when bootstrapping from GHC 8.2.1, but this is still a long time away.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">Alan<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">On 13 November 2017 at 15:05, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">That’s not a problem you are trying to solve – it’s just some code
</span><span style="font-size:12.0pt;font-family:"Segoe UI Emoji",sans-serif">😊</span><span style="font-size:12.0pt">.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Let me hazard a guess. You want a derived instance for</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> Data (OverLit (GhcPass p))</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">But to do that of course we’ll need Data (XEOverLit (GhcPass p)).</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">We can’t possibly have that at the time of writing the instance declaration, because p is universally quantified. So we are stuck with</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> instance (Data (XEOverLit (GhcPass p))) => Data (OverLit (GhcPass p)) where…</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">and the context gets a constraint for every extension field in OverLit, which is painful.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">So we have a solution bit contexts, but it’s painful.
</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Is that the problem you are trying to solve?</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Simon</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Alan & Kim Zimmerman [mailto:<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>]
<br>
<b>Sent:</b> 13 November 2017 13:01<br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>></span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b>Subject:</b> Re: Trees that Grow and constraints<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt">Sorry, I simplified the problem.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt">The actual one I am trying to solve will be the general case, so e.g.<br>
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type instance XEOverLit (GhcPass 'Parsed ) = RdrName<br>
type instance XEOverLit (GhcPass 'Renamed ) = Name<br>
type instance XEOverLit (GhcPass 'Typechecked) = Id<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt">or more likely, modelling the existing PostTc and PostRn usages.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt">And I suppose looking into using them in a single definition might work<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">Alan <o:p>
</o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">On 13 November 2017 at 14:55, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
Or do I misunderstand your advice?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Well, you said
</span><o:p></o:p></p>
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Where specifying the type instance with a wild card keeps GHC happy (the XXOverLit case), but specifying for each of the three constructors for pass does not (the XEOverLit case)<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">My question is: why not just use the wildcard, in that case?</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Do you want to re-state the problem you are trying to solve?</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Simon</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Alan & Kim Zimmerman [mailto:<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>]
<br>
<b>Sent:</b> 13 November 2017 12:49<br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>><br>
<b>Cc:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Re: Trees that Grow and constraints</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">So why not use one?
</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">Simon</span><o:p></o:p></p>
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</blockquote>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<div>
<div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt">If I do<br>
<br>
instance (Data p) => Data (Experiment p)<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt">then GHC does not know that the type instances for
<br>
<br>
<br>
type instance XEOverLit (GhcPass 'Parsed ) = PlaceHolder<br>
type instance XEOverLit (GhcPass 'Renamed ) = PlaceHolder<br>
type instance XEOverLit (GhcPass 'Typechecked) = PlaceHolder<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">apply.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Or do I misunderstand your advice?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Alan<o:p></o:p></p>
</div>
</div>
</div>
</div>
</div>
</div>
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</blockquote>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
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