custom module to run alongside feed import

Hi,

I want to run a series of validation and check functions before the feed creates/saves the created node.
Im writing a custom module with these functions in, and im wondering how to incorporate these into feeds module?

(eg: if a child 'type' with 'Lease' in it, ignore and dont create a node)
Im using the xpath parser as im importing an XML feed and have mapped the fields in the settings.

Any advice how to do this would be greatly appreciated!
(sorry if i sound vague or dense, feeds is new territory for me!)