module email
sig Domain, Target {}
disj sig Name extends Target {}
disj sig Addr extends Target {dom: Domain}
part sig Alias, Group extends Name {}
sig AddrBook {
	map: Name -> Target
	}{all a: Alias | sole map[a]}
fun getDomain (b: AddrBook, n: Name): set Domain {
	result = n.^(b.map).dom } 
run getDomain