Pasting primary selection from keyboard

A few years ago I wrote about a setup I coded for myself for when I want to make paying my bills a tiny bit less painful. One thing I mentioned then was the primary selection. For those of you who don’t know it: under X.org, you can select a portion of text (for example by dragging the mouse over it) and then paste it in the same or another application with clicking the middle button. It is extremely useful for quick copy-and-paste operations where you do not have to issue a separate “copy” command (usually C-c, but often also found on mouse context menu). And by the way, Wayland also supports this. Sometimes I find it useful to use it even without the mouse.