Bind the given mouse button within the contexts to invoke proc. contexts is a list of event-contexts (e.g., '(button1 sidebar)) button is a string or integer giving the mouse button number proc is a procedure that will be invoked (with no arguments) when the specified button is pressed in the specified context. See bind-key for a list of the contexts. If immediate-proc is given, it will be executed immediately on a button-click. If immediate-proc returns #f, then proc will still get executed after determining the mouse click type.