[openbox] Do some action even if keychain is not complete

[XXXX]

Hello everybody! I would like to implement the following functionality:
Meta+Left - window is placed in the left half of the screen
Meta+Left+Up - window is placed in the left top quarter of the screen
Meta+Left+Down - window is placed in the left bottom quarter of the screen etc.
How can I do so? Meta+Left+Up/Down works, but I can not add action for
Meta+Left.
Thank you.