Looking for list comprehensions use cases

[Jan Stolarek]

Haskellers,

recently I've been looking into the possibility of creating some new optimisations for GHC. These 
would be mostly aimed at list comprehensions. Here's where I need your help:

1. Do you have complex list comprehensions usage examples from real code? By complex I mean  
nested list comprehensions, reading from more than one list ([ ...| x <- xs, y <- ys ... ]) etc.

2. Do you have list comprehensions code that you had to optimize by hand because GHC was unable to 
make them fast enough?

Janek