Almost all problems (that I find interesting): anytime you can frob two foos together and get a new foo (put two parts together to make something that can again

Almost all problems (that I find interesting): anytime you can frob two foos together and get a new foo (put two parts together to make something that can again be a part) you've got the material for a tree. Concrete examples: Docker containers, PDF parts, POSIX filesystems, Scheme sources, SVGs, x86 objects, etc.