diff --git a/plt/README.md b/plt/README.md
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+++ b/plt/README.md
@@ -1,3 +1,9 @@
+# Programming Language Theory
+
 * [Can Programming Be Liberated from the von Neumann Style? A Functional Style and Its Algebra of Programs](http://www.thocp.net/biographies/papers/backus_turingaward_lecture.pdf)
 
 * [Programming and Reasoning with Algebraic Effects and Dependent Types](http://eb.host.cs.st-andrews.ac.uk/drafts/effects.pdf)
+
+* [Backtracking Iterators](https://www.lri.fr/~filliatr/publis/enum2.pdf)
+
+* [Breadth-First Numbering: Lessons from a Small Exercise in Algorithm Design](http://www.cs.tufts.edu/~nr/cs257/archive/chris-okasaki/breadth-first.pdf)
diff --git a/testing/README.md b/testing/README.md
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+# Testing
+
+## QuickCheck
+
+[QuickCheck: A Lightweight Tool for Random Testing of Haskell Programs](http://www.cs.tufts.edu/~nr/cs257/archive/john-hughes/quick.pdf)
+
+[QuickCheck Testing for Fun and Profit](http://people.inf.elte.hu/center/fulltext.pdf)
+
+Property-Based Testing, best explained in John Hughes own talk at [ClojureWest 2014](https://www.youtube.com/watch?v=zi0rHwfiX1Q&list=PLZdCLR02grLp__wRg5OTavVj4wefg69hM).
diff --git a/user_interfaces/README.md b/user_interfaces/README.md
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+# User Interfaces
+
+[Squeak: a Language for Communicating with Mice](http://ordiecole.com/squeak/cardelli_squeak1985.pdf)