 20,
            
            source: { type: "gpio", gp: 7 },
            action: { type: "xInput", button: "a" },
         },
      ],
   }
</pre>

<p>
   That defines the button wired to GPIO 7 as a modifier button, with shift
   bit 0x0001.  Whenever this button is being pressed, bit 0x0001 is added
   (or, rather, "OR"'d) into the "global shift state".  Other buttons can
   interrogate the global shift state when pressed, and activate or <i>not</i>
   activate according to whether certain bits are set.