Emacs tip of the day: kill-do-not-save-duplicates prevents pushing duplicates to the kill-ring, so you don't need to press M-y repeatedly.
miniblog.
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misc.el contains a number of handy Emacs commands. For example, copy-from-above-command duplicates the non-blank line above point.
I'm also still getting used to Smalltalk-style docstrings. They're 1st person, so eg Set "I represent a set of objects without duplicates".