Function take_seed 

pub(crate) fn take_seed(seed: &mut Option<usize>, own: usize) -> Option<usize>

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

The walk is a single flat index over every free parameter reachable from a curve, in the order parameter_owners reports them. Each curve consumes its own parameters off the front: if the index falls inside them the seed lands here, otherwise the index moves past them and the curves it reads get their turn.