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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">OK – a good thread. From it I have learned<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span style="mso-fareast-language:EN-US">What the goal is<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span style="mso-fareast-language:EN-US">That exposing (~#) and (~R#) is not just lexically tricky; it’s deeper than that (Richard’s note below).<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span style="mso-fareast-language:EN-US">That there is a viable, implementation plan; not a thing of beauty perhaps, but ok if carefully documented<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Sounds good to me!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">My suggestion, if someone is motivated enough to pursue this:<o:p></o:p></span></p>
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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo2"><span style="mso-fareast-language:EN-US">write a wiki page to summarise the above points<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo2"><span style="mso-fareast-language:EN-US">plus a ticket to track progress<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo2"><span style="mso-fareast-language:EN-US">implement the design<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Happy to help review.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><br>
Sorry to have been slow to understand.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><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"> Richard Eisenberg <rae@cs.brynmawr.edu>
<br>
<b>Sent:</b> 05 September 2018 16:26<br>
<b>To:</b> Ryan Scott <ryan.gl.scott@gmail.com><br>
<b>Cc:</b> Simon Peyton Jones <simonpj@microsoft.com>; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Unpacking coercions<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">I don't think that wiki reference is really about this problem.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Instead, I think that we'd need Constraint# to be able to offer users ~# and ~R#. The problem, then, is that there is no answer to this question: Constraint is to Type as Constraint# is to what? Currently, if ~# and ~R# are the only two
generators of Constraint#, then Constraint# is like TYPE (TupleRep '[]). But I think wiring that in would be short-sighted.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Instead, this road leads to CONSTRAINT :: RuntimeRep -> Type, which is like TYPE. Today's Constraint would be CONSTRAINT LiftedRep. And we would have (~#), (~R#) :: forall k1 k2. k1 -> k2 -> CONSTRAINT (TupleRep '[]). In the Glorious Future,
we'll have CONSTRAINT ~R TYPE, but we're not there yet. <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">This all feel like the Right Answer to me, but it's unclear if we should start down this road before we have roles in kinds.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal">On Sep 5, 2018, at 10:54 AM, Ryan Scott <<a href="mailto:ryan.gl.scott@gmail.com">ryan.gl.scott@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">> We could utterly lie and say<br>
> <br>
> data Coercion a b where<br>
> <br>
> Coercion :: Coercion a a<br>
> <br>
> <br>
> <br>
> That would generate the right bits in in the .o file, and we’d totally ignore the .hi file. Gruesome but I think it would work.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">This was precisely what I had in mind. We already perform this trick for several things defined in ghc-prim (e.g., the (~) and (~~) classes), so this would just be another example of that.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:10.0pt">> But rather than all this circumlocution, why don’t we just make it possible to write ~# and ~R# directly. Even if we dodge the need right now, it’ll surely come back.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hah, the reason David is suggesting his idea in the first place is to avoid having to do this! My understanding is that making it possible to write (~#) and (~R#) directly would involve quite a number of complications, as Richard's wiki
entry on the subject [1] demonstrates.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">That's not to say that I wouldn't like to see that happen some day. But a mere mortal like myself couldn't possibly implement this, whereas David's idea is actually within reach.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Ryan S.<o:p></o:p></p>
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<p class="MsoNormal">-----<o:p></o:p></p>
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<p class="MsoNormal">[1] <a href="https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Internal#Liftedvs.Unliftedequality">
https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Internal#Liftedvs.Unliftedequality</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Wed, Sep 5, 2018 at 10:45 AM, 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">
I think the intention is to have that proposal (which proposes a language change) be superseded by this idea (which does not change the language).<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">Oh, I did not know that. I’ll ignore the proposal for now, 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"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt;margin-left:36.0pt">
All that would take is putting Coercion in TysWiredIn, and moving Coercion from Data.Type.Coercion to somewhere in ghc-prim.<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 don’t think it’s quite as simple as that. Yes, we can wire it into the compiler; but we still need a module that defines the info table, curried
data constructor etc for the type. And we have no way to do that.</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 could utterly lie and say</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 Coercion a b 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"> Coercion :: Coercion a a</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">That would generate the right bits in in the .o file, and we’d totally ignore the .hi file. Gruesome but I think it would work.</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>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:12.0pt">But rather than all this circumlocution, why don’t we just make it possible to write ~# and ~R# directly. Even if we dodge the need right now, it’ll
surely come back.</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">If that is lexically tiresome, we could I suppose provide builtin-aliases for them, as if we had</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"> type NomEq# = (~#)</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"> type ReprEq# = (~R#)</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"> Ryan Scott <<a href="mailto:ryan.gl.scott@gmail.com" target="_blank">ryan.gl.scott@gmail.com</a>>
<br>
<b>Sent:</b> 05 September 2018 15:26<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: Unpacking coercions</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">
Simple is good. But what is this dead simple idea?</span><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'm referring to David's first e-mail on this thread:
<a href="https://mail.haskell.org/pipermail/ghc-devs/2018-September/016191.html" target="_blank">
https://mail.haskell.org/pipermail/ghc-devs/2018-September/016191.html</a><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">All that would take is putting Coercion in TysWiredIn, and moving Coercion from Data.Type.Coercion to somewhere in ghc-prim.<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">> <span style="font-size:12.0pt">
Maybe this thread belongs with the proposal, unless I’m misunderstanding.</span><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 think the intention is to have that proposal (which proposes a language change) be superseded by this idea (which does not change the language).<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">Ryan S.<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 Wed, Sep 5, 2018 at 10:20 AM, 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">Simple is good. But what is this dead simple idea?</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">Perhaps:
<a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F116&data=02%7C01%7Csimonpj%40microsoft.com%7C51cff02dc13341258db108d61343d184%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636717579340406159&sdata=1VQIATP4YeqvffmUsqIE6m5aDOe55nlf4NE6WOqWZ%2BU%3D&reserved=0" target="_blank">
https://github.com/ghc-proposals/ghc-proposals/pull/116</a></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 that proposal lists several possible alternatives. Which one did you mean?</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 all of them are language changes. Making evidence strict would require no language changes to solve the original problem.</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">Maybe this thread belongs with the proposal, unless I’m misunderstanding.</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"> ghc-devs <<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@haskell.org</a>>
<b>On Behalf Of </b>Ryan Scott<br>
<b>Sent:</b> 05 September 2018 15:15<br>
<b>To:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Re: Unpacking coercions</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">These aren't mutually exclusive ideas. While I'm sure there's many ways we could solve this problem, David's idea has the distinct advantage of being dead simple. I'd rather not block
his vision on some other large refactor that may never materialize. (And if it _does_ materialize, we could revert any wiring-in of Coercible quite easily.)<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">Ryan S.<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"><o:p> </o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
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