That's something that confused me.  How do I assign/declare mat to be the material?  Does Python know that 

for mat in mats

means that "mat" means "whatever is lower in the tree from mats(defined)?"

 I think I need a good flowchart of the hierarchy in order to get the recursive stuff sorted out.  Or I may just be over thinking it, if my above assumption is correct. 

Sorry about the indentation, when I did copy/paste it didn't keep the formatting.  I'll be careful of that.  The formatting was correct in my text editor, though.