Clojurians Log v2

if i need to setval at nth position counted from back, any suggestion how to do it? i.e. something like

(setval [(before-index -2)] :x [1 2 3 4 5 6 7])
; [1 2 3 4 5 :x 6 7]

which of course doesn't work because -2 (negative) index isn't interpreted as count from back. do i need to create a new navigator or is there a way to compose something out of existing ones?