Function pins_reach_chain 

pub fn pins_reach_chain(
    index: &Index,
    pin_depth: usize,
    chain_position: Option<usize>,
) -> bool

Whether a point read pinning the first pin_depth properties of index may stop at the deepest pin’s value tree: the pins reach the chain starting at chain_position (the shallowest ranked at level, whose value trees and every one below aggregate their whole subtree) and leave a deeper property free. The count and sum pickers and path builders all ask this, each with its own chain position, so a picker never admits what its builder refuses.