For Windows and 2 button mice, `` is the left button and
`` is the right button.  You may have an OS extension which
emulates the 2nd button by pressing both buttons simultaneously.

For the Mac, ???

For those who lack ``, the only core bindings made for that
are fast scanning of listbox, entry and text widgets.  Programmers with
potential users in this category should be sensitive to this loss and
try and offer alternatives (like ``).