How to get length of foo[1] in example below in assembly?

uint[][] foo; // foo.length = 2, foo[1].length = 3

function getLength() returns (uint len) {
    assembly {
        len := sload(foo) // returns 2
        len := sload(........) // how to return foo[1] ?

This guide explained it fairly well. The storage pointers for nested arrays are organized as nested hash functions. At level one, keccak256(foo_slot), are the lengths for all first order arrays, foo[0], foo[1], foo[2], etc. The length of each individual subarray is, correspondingly, keccak256(foo_slot), keccak256(foo_slot) + 1, keccak256(foo_slot) + 2, etc.

In the example asked about, the length of foo[1] is found with:

len := sload(add(keccak256(foo_slot, 32), 1)
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