// Register fact types with gob.
	// In VetxOnly mode, analyzers are only for their facts,
	// so we can skip any analysis that neither produces facts
	// nor depends on any analysis that produces facts.
	//
	// TODO(adonovan): fix: the command (and logic!) here are backwards.
	// It should say "...nor is required by any...".
	//
	// Also build a map to hold working state and result.
	type action struct {
		once        sync.Once
		result      interface{}
		err         error
		usesFacts   bool // (transitively uses)
		diagnostics []analysis.Diagnostic
	}
	actions := make(map[*analysis.Analyzer]*action)
	var registerFacts func(a *analysis.Analyzer) bool
	registerFacts = func(a *analysis.Analyzer) bool {
		act, ok := actions[a]
		if !ok {
			act = new(action)
			var usesFacts bool
			for _, f := range a.FactTypes {
				usesFacts = true
				gob.Register(f)
			}
			for _, req := range a.Requires {
				if registerFacts(req) {
					usesFacts = true
				}
			}
			act.usesFacts = usesFacts
			actions[a] = act
		}
		return act.usesFacts
	}