Make a start on Spin groups as an article

templates/block.tree

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+\import{macros}
+
+\block{}{
+
+
+
+}

trees/spin-0001.tree

@@ -11,11 +11,13 @@
 
 \date{2024-04-26}
 
-\p{This survey is built on my notes in the process of figuring out [[ericwieser]]'s MathOverflow question [Definition of a spin group](https://mathoverflow.net/questions/427881/definition-of-a-spin-group) for [our PR to Mathlib4 about Spin groups](https://github.com/leanprover-community/mathlib4/pull/9111/).}
+\transclude{spin-000R}
 
-\p{This is also my first [[uts-0001]].}
+\transclude{spin-000S}
 
-\p{Now it's a work-in-progress. Definitions coming from different sources are simply quoted here with minimal modifications, to include immediate prerequisites, and omit some discussions or theorems. Definitions that are too similar are not yet pruned. And the order is quite arbitrary at the moment.}
+\block{Appendix: definitions of Spin group from various source}{
+
+\p{Definitions coming from different sources are simply quoted here with minimal modifications, to include immediate prerequisites, and omit some discussions or theorems. Definitions that are too similar are not pruned. The order is quite arbitrary.}
 
 \transclude{spin-0002}
 
@@ -65,4 +67,6 @@
 
 \transclude{spin-000P}
 
-\transclude{spin-000Q}
+\transclude{spin-000Q}
+
+}

trees/spin-000R.tree

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+\import{macros}
+
+\title{Motivation}
+
+\p{This survey is built on my notes in the process of figuring out [[ericwieser]]'s MathOverflow question [Definition of a spin group](https://mathoverflow.net/questions/427881/definition-of-a-spin-group) for [our PR to Mathlib4 about Spin groups](https://github.com/leanprover-community/mathlib4/pull/9111/).}
+
+\p{This is also my first [[uts-0001]].}

trees/spin-000S.tree

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+\import{macros}
+
+\title{Status}
+
+\p{Currently it's a work-in-progress.}
+
+\p{TODO}
+
+\ul{
+    \li{classify them by the prerequisites, style, and similarity}
+}