exo4: add a generic Node class and make use of it.
-rw-r--r-- 224 .gitignore
drwxr-xr-x - exo1
drwxr-xr-x - exo2
drwxr-xr-x - exo3
drwxr-xr-x - exo4
drwxr-xr-x - exo5
drwxr-xr-x - exo6