Report with subreports (multiple reports)

Hi,

How do I compose large reports from individual smaller ones?

I.E I have created 2 report designs: rp1.rptdesign and rp2.rptdesign and I would like to make a new "main" report containing them both.

In the gui it is done by copy-pasting the elements from the design of the subreport to the design of the main report.
How do I do it in runtime? What is the API?
Any code examples?

Thanks.

Tal.