Plum does not ignore the top level directory of what I have specified

When i do that

mkdir -p a/b/c
mkdir -p a/d/e
find ./a -path ./a/d -prune -o -type d

I understood:

./a
./a/d
./a/b
./a/b/c

I must misuse the prune flag. How to ignore the directory ./a/d as well at the end of my discovery?