Constant ENTRY 

const ENTRY: u64 = u64::MAX; // 18_446_744_073_709_551_615u64

The caller of a root span.

A synthetic key in the same space as the real ones, which is a collision at 2^-64 per distinct resource and not worth a tagged enum on every edge. The obvious free value is crate::identity::NO_IDENTITY, and it is taken: zero already means undescribed resource, and merging “traffic from outside” into “resources nobody labelled” is the one confusion a service map must not have.