[cairo] How to copy one cairo_t's context to another?

[Magicloud Magiclouds]

Dear,
    I created a cairo_t from: layer = 
cairo_create(cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 
rect->width, rect->height)). And draw something on it. Then I want to 
copy them to the main cairo_t. How to do that?

Thanks.