diff --git a/src/Investing-and-trading.md b/src/Investing-and-trading.md
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@@ -1,6 +1,7 @@
 % Investing and trading
 
-- [Beancount: Inventory Booking](https://docs.google.com/document/d/1F8IJ_7fMHZ75XFPocMokLxVZczAhrBRBVN9uMhQFCZ4), [discussion](https://groups.google.com/forum/#!searchin/ledger-cli/inventory/ledger-cli/aQvbjTZa7HE/x3KNPteJWPsJ)
+- [Beancount: Inventory Booking](https://docs.google.com/document/d/1F8IJ_7fMHZ75XFPocMokLxVZczAhrBRBVN9uMhQFCZ4) (2014), [discussion](https://groups.google.com/forum/#!searchin/ledger-cli/inventory/ledger-cli/aQvbjTZa7HE/x3KNPteJWPsJ)
+- [Beancount: Calculating Portfolio Returns](http://furius.ca/beancount/doc/returns) (2020)
 - [hledger: Track investments](https://hledger.org/track-investments.html)
 - [hledger: Track investments 2](https://hledger.org/investments.html)
 - [[Restricted stock units]]