seen := make(map[string]bool)
	// apply environment settings
	parsegodebug(env, seen)
	// apply compile-time GODEBUG settings for as-yet-unseen variables
	parsegodebug(godebugDefault, seen)
	// apply defaults for as-yet-unseen variables
	for _, v := range dbgvars {
		if v.atomic != nil && !seen[v.name] {
			v.atomic.Store(0)
		}
	}
}

// parsegodebug parses the godebug string, updating variables listed in dbgvars.