Make Apple CMD/Command key an additional CTRL/Control key in X11

.Xmodmap

! clear CMD of it's regular task, whatever that might be
clear mod4

! configure CTRL and CMD to act as left Control
keycode 133 = Control_L NoSymbol Control_L
keycode 37 = Control_L NoSymbol Control_L

! use CTRL and CMD to act as left Control
add control = Control_L

autoload_xmodmap.sh

echo "exec xmodmap ~/.Xmodmap &" >> .i3/config

find_correct_keycode

➜  ~  xmodmap -pke | grep Super_L
keycode 133 = Super_L NoSymbol Super_L

load_xmodmap.sh

xmodmap ~/.Xmodmap