Where to learn how to use mattes?

Hi again! I have a problem blender can solve but i cant find any information on how to do it and cant find any good tutorials. I have a part of one renderlayer which is totally green (NOT the background, just an area of it). I want to replace this area with the output from another render layer. In video editing programs this would usually be done via the “color keying” effect. And indeed there is such a node in blender… BUT its only got one image input! How can it blend stuff together without a second video input?

I gather blender calls this keying stuff “mattes”, so, anyone know any good tutorials for mattes? I found a page from a blenderwiki but its from an older version and no longer works.

You mix them in another node. The colour key node only changes the input colour to transparent, then you can use an AlphaOver node to mix it with another image.