I has been a while since I even touched the editor, Dr. :D
Frizi, thanks for the demonstration. I did everything precisely in the same manner, again, and it worked!
The problem I was afraid of popped up. the extra minerals are supposed to come as a BONUS besides the normal amount. This means that a tree yields one hundred minerals.
I guess that means either the bonus has to be added just as the worker returns the lumber, or that the tries have to hold the extra recources ,and must be destroyed as soon as someone is done mining them. Eh. I'll see what I can do with behaviours or triggers. Thanks, fellas. :)