An interesting Scala for comprehension example (by Daniel Sobral)

I just ran across the following Scala for comprehension example, and thought it was a very good example:

for {
  b1 <- books
  b2 <- books
  if b1 != b2
  a1 <- b1.authors
  a2 <- b2.authors 
  if a1 == a2
} yield a1