Function take_params 

pub(crate) fn take_params<'a, R: Scalar>(
    rest: &mut &'a [R],
    n: usize,
) -> &'a [R]

One step of a substituted walk over a composition’s parameters.

Takes this curve’s own n parameters off the front of rest and leaves the remainder for the curves it reads, in the order parameter_owners reports them — the same order a seeded index counts in.

The returned slice is borrowed, so substituting costs no allocation at any depth. This is what replaces arming: a reverse-mode risk pass registers one flat vector of handles, hands it here, and the storage is never touched.