using a v157 base install.... i'm working on new admin plugin...

lets say there i have a class file, that is in:

zc_plugins/myplugin/1.0.0/admin/includes/classes/class.php

i now want to require() a new script in this class file. previously i might do something like

PHP Code:
require_once DIR_FS_ADMIN .  'includes/classes/other_class.php'; 
when i look at the $_SERVER vars, available to me, nothing pinpoints where i am in the file system to refer to new files that ideally i would like to keep in my encapsulated plugin.

is there an easy way to accomplish this goal? i'm guessing there is a ZC var that knows where i am. as i do not...

thanks in advance.