If I understand correctly, for signed measures $\nu,\mu$ and $\lambda$ on a common measurable space, if $\nu$ and $\mu$ have enough null sets, then $$\frac{d\nu}{d\lambda} = \frac{d \nu}{d\mu}\frac{d \mu}{d\lambda},$$ $\lambda$-almost everywhere.
This looks suspiciously like the Leibniz chain rule, except that, well, the Leibniz chain rule doesn't make too much sense to me, whereas the above formula is perfectly clear. Anyway, I was wondering if its possible to recover the Langrange-notation version of the chain rule from single-variable calculus, namely $$(g \circ f)' = (g' \circ f) \cdot f',$$ from the above formula.