module addressBook

abstract sig Target {}
sig Addr extends Target {}
abstract sig Name extends Target {}

sig Alias, Group extends Name {}
sig Book {addr: Name -> Target}

pred showMe () {some addr}
run showMe for 3

fact {all b: Book | no n: Name | n in n.^(b.addr)}
-- fact {all b: Book, a: Alias | sole b.addr[a]}