map
Find the key/value pair in this map which has the specified key and answer the value. Fail if the specified key is not present in the map. The result is not forced to be immutable, as it's up to the caller whether the new reference would leak beyond a usage that conserves its reference count.
Return
The value associated with that key in the map.
Parameters
key
The key to look up.
Throws
If the key was not found.