Instead of just tossing this bit of history in the trash I decided to scan and post it for posterity. In the mid-80s I worked for EMS/McGraw-Hill on a series of products for the Gregg division that were used to train students on how to use Lotus 1-2-3 and spreadsheets in general. We created a product called McGraw-Hill Integrated Software (MHIS) which included a spreadsheet module (written in C for MS-DOS). The memo I wrote explaining the implementation of natural order evaluation in a spreadsheet vs. column-order evaluation in a spreadsheet is below. It was fun to see that I was able to use my computer science education to bring a postorder traversal of a binary tree to the project!