Interrupts

Lisp - Dispatch
Haskell - Timer
Scheme - Start I/O
HTML - Wait on Semaphore
Java - I/O Interrupt
Python - Trap
C - Access Granted by Semaphore
C# - Sleep
C++ - Exit
Assembly - Created
Ruby - Swap In
Prolog - Swap Out

>>11
The hierarchy is the most efficient way of obtaining information:

1. Find out which artist made the song you want to listen to.
2. Find out the title of the song.
3. Search the hierarchy.

As there are fewer artists than songs, a hierarchy that's built the other way would be slower.