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@@ -6,19 +6,39 @@
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diff --git a/Concurrent Systems/notes/9 - Universal Object.md b/Concurrent Systems/notes/9 - Universal Object.md
new file mode 100644
index 0000000..a2a80f4
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+++ b/Concurrent Systems/notes/9 - Universal Object.md	
@@ -0,0 +1,11 @@
+Given objects of type T and an object of type Z, is it possible to wait-free implement Z by using only objects of type T and atomic R/W registers?
+
+If yes, then Z is not as essential as T (T can do everything Z can do).
+
+But what is the **type** of an object?
+1. the set of all the possible *values for states* of objects of that type
+2. a set of *operations* for manipulating the object, each provided with a *specification*: a description of the conditions under which the operation can be invoked and the effect of the invocation
+
+We focus on types whose operations are:
+- **Total:** all operations can be invoked in any state of the object
+- **Sequentially specified:** given the initial state, the behavior depends only by the sequence of operations, where the output to every operation invocation only depends on the input arguments and the invocations preceding it.
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