"A [[Map]] with exactly one [[entry]]." see (`class SingletonSet`, `class Singleton`) shared serializable class SingletonMap<Key, Item>(shared Key->Item entry) satisfies Map<Key, Item> given Key satisfies Object { defines(Object key) => entry.key==key; shared actual Item? get(Object key) { if (entry.key==key) { return entry.item; } else { return null; } } contains(Object entry) => this.entry == entry; iterator() => Singleton(entry).iterator(); equals(Object that) => (super of Map<Key, Item>).equals(that); hash => (super of Map<Key, Item>).hash; each(void step(Key->Item element)) => step(entry); shared actual SingletonMap<Key,Item> clone() => this; }