Returns the value at the specified key in the current
public function at( Tk $k, ): Tv;
If the key is not present, an exception is thrown. If you don't want an
exception to be thrown, use
$v = $map->at($k) is semantically equivalent to
$v = $map[$k].
Tk $k- the key from which to retrieve the value.
Tv- The value at the specified key; or an exception if the key does not exist.
Map::at for usage examples.