For objects passed by reference, a Swift "let" signifies immutability of reference (not value); thus you can append to the NSMutableArray but not reassign to a

For objects passed by reference, a Swift "let" signifies immutability of reference (not value); thus you can append to the NSMutableArray but not reassign to a different NSMutableArray. On the other hand a native Swift Array assigned via "let" would be immutable from both a value and assignment perspective.