Function derive 

pub fn derive(o: &Observations) -> Preflight

The preflight these observations support. Reads nothing.

use majordomus_cli::environment::preflight::{derive, DeploymentObservation, GitObservation, Observations, Verdict};
let head = "b".repeat(40);
let mut o = Observations::empty("demo", 0);
o.git = Some(GitObservation { branch: None, head: Some(head.clone()), clean: true, changed: 0 });
o.deployment = DeploymentObservation::Published { commit: "d".into(), at: "t".into(), source: Some(head) };
assert_eq!(derive(&o).check("verification.deployment").unwrap().verdict, Verdict::Verified);