Summary: Use "home row" keys as modifiers (shift, hyper, super, kana,roya)
This is a filter which parses the stream of keyboard events and detects simultaneous key-presses to reinterpret specific use of selected keys as modifiers, instead of their regular function. Also the timing is significant.
Example: use "a" key to activate numeric keypad on 'uio jkl m-." keys.
Packaged as a plugin for X server, Weston plugin, and a Windows 10+ filter kernel driver.
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Windows ... see instructions
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Linux -- either build from source, or use Debian (Sid) packages from my reprepro apt repository