Function compose 

pub fn compose(
    frames: &[StackFrame],
    index_by_label: &HashMap<String, usize>,
) -> Vec<StackResult>

Compose per-function profiles into a per-function maximum over the call tree: total(f) = own(f) + MAX over call sites of total(callee).

index_by_label maps a direct-call label (func_<idx>) to a position in frames; a label absent from it is an external callee and declines.