doc: /* t means that when the option-key is held down while pressing the
mouse button, the click will register as mouse-2 and while the
command-key is held down, the click will register as mouse-3.
-'reverse means that the the option-key will register for mouse-3
+'reverse means that the option-key will register for mouse-3
and the command-key will register for mouse-2. nil means that
no emulation should be done and the modifiers should be placed
on the mouse-1 event. */);