A key array represents all the keyframe keys in a MAX controller track.
$foo.position.keys
$foo.twist.angle.keys
the key array is always constructed by accessing the keys property on an animatable MAX object property
<key_array>[<integer>]
<key_array>[<integer>] = <key>
append <key_array> <key>
delete <key_array> <number>
<key_array>.count