f(n) = n \times f(n-1) where n is any natural number greater than 0.
The recursive nature of stories which borrow from each other.
HURD is an co-recursive acronym, standing for HIRD of Unix-Replacing Daemons, where HIRD stands for HURD of Interfaces Representing Depth.
Co-recursive algorithms allow infinite large objects to be used without necessarily running out of memory.