Function evaluate_selection 

pub fn evaluate_selection(
    gate: &SelectionGate,
    baseline: &Baseline,
    baseline_evidence: &RepeatEvidence<'_>,
    candidates: &[NamedCandidate<'_>],
) -> Result<Selection, String>

Verify all supplied bindings before comparing values. A failed precondition makes that candidate unknown. Once preconditions hold, any definite clause failure dominates unknown clauses, including unknown coverage denominators.