This example is weirdly spelled:

    echo '{"k1": [{"k2": [9]}]}' | jq '.k1 | .[0] | .k2 | .[0]'

is equivalent to

    echo '{"k1": [{"k2": [9]}]}' | jq '.k1[0].k2[0]'

I kept waiting for the author to explain this, but they did not