Return a represention of the next mouse event. The return value is (string modmask button-number #t). The cdr of the return value can be used as the arguments to undo-passive-grab and redo-passive-grab. The string is usable as a mouse binding string. Modifiers are listed first, separated by "-" followed by a "-" and the button number. E.g., "S-C-M-1" is Shift+Control+Meta + button 1. See also get-next-event.