[Therion] MetaPost include relative path

Hi, Whats the syntax for including code from a sub directory in MetaPost? My project has a subdirectory of code snippets in {doc root}\code The MetaPost input command works great when I reference the entire directory location eg (input "c:\users\henry\documents\survey\cave\code\filename") but won't accept any form of relative address that I can see. Doesn't make it very portable. Any ideas Thanks Henry