The - operator now negates its operand when it is called with exactly one operand.
When it is called with two or more, all of the remaining operands are subtracted
from the first (this was the preexisting behavior).
Use objectForKey:withDefault: to lookup an object and return a specified default if no object is found.
Use each: to iterate over the key-object pairs in a dictionary. Pass it a block with two arguments: (key object).