Programmatically checking checkboxes

I've written a module that grabs data from an XML doc and creates a node out of it. In my node type I have checkboxes. I have made the relevant information appear in one of the xml fields as a comma-separated list, which i explode and then populate the checkboxes appropriately. This is where I'm stuck. I see the checkboxes take the form field_thisfield[keys][element], with value 'element' on the standard submit-node form. How do I deal with this programmatically? How can I check the appropriate boxes while creating the node through my module?