diff --git a/doc/notes.rst b/doc/notes.rst
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+++ b/doc/notes.rst
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+Random notes.
+
+Type inference.
+
+Expressions represent parsed abstract syntax trees.
+We use the same data structure to represent syntax trees both before and after
+type inference.
+
+We record each Expression's type using a type_ref instance.
+A type_ref does two things:
+1. gives the expression's type a unique name
+2. records expression's concrete type once type infrerence has completed.
+
+Type inference resolves each Expression's type to a concrete TypeDescr.
+An Expression tree can't be compiled unless all reachable Expression nodes
+are resolved to concrete types, i.e. type inference has completed successfully.
diff --git a/include/xo/expression/typeinf/type_ref.hpp b/include/xo/expression/typeinf/type_ref.hpp
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+++ b/include/xo/expression/typeinf/type_ref.hpp
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+/** @file type_ref.hpp **/
+
+#pragma once
+
+namespace xo {
+    namespace scm {
+        using type_var = xo::flatstring<11>;
+
+        /** @class type_ref
+         *  @brief name and eventual resolution for type associated with an expression.
+         *
+         *
+         **/
+        struct type_ref {
+
+
+        };
+    } /*namespace scm*/
+} /*namespace xo*/
+
+/** end type_ref.hpp **/