The proper way to parse or construct nested list comprehensions as explained in pep 202 is to think like you're using normal for:

for a in b:

  for c in a:

    use(a,b,c)

 

This would be

[use(a,b,c) for a in b for c in a]

Everything stays the same except the "use" part that goes in the front (the rule also includes the filters - if).