xev ouput for the left ALT key (updated):

KeyRelease event, serial 37, synthetic NO, window 0x2600001,
    root 0x170, subw 0x0, time 833473, (163,-12), root:(794,345),
    state 0x88, keycode 64 (keysym 0xfe03, ISO_Level3_Shift), same_screen YES,
    XLookupString gives 0 bytes: 
    XFilterEvent returns: False