I can make sense of nested Python list comprehensions if I read them as shorthand for nested for loops, in which case you need to define variables in the outer loop before you can define dependent variables in the inner loop.

e.g. with your example above,

  for xs in xss:
      for x in xs:
          x

What's confusing about the syntax is that `x` in your example above comes first, whereas in actual nested loops it comes last. So maybe:

  [for xs in xss for x in xs : x]

Could be alternative syntax that moves `x` to the end?