[Exim] System Filter - Multiple Files based on a lookup

I must be going crazy. I know that somewhere in the exim doc I saw an
example of using a different filter file based on a lookup. I thought
it was a system filter, but I searched spec.txt and filter.txt for EVERY
occurrence of filter with no luck.

So, can a system filter be specified as an expression that resolves to
the actual file path. It would really help neaten up my current
project. The filter is getting really big.