void popup(int x, int y, int mouse_button, int time);
Pops up the complete menu at (x, y).
The mouse_button parameter should be the mouse button pressed to initiate the menu popup.
If the menu popup was initiated by something other than a mouse button press,
such as a mouse button release or a keypress, mouse_button should be 0.
忘却曲線を使ってこの知識を確実に記憶に残す
