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/**
 Provides the ability to verify key paths at compile time.
 
 If "keyPath" does not exist, a compile-time error will be generated.
 
 Example:
    // Verifies "isFinished" exists on "operation".
    NSString *key = SQKeyPath(operation, isFinished);
 
    // Verifies "isFinished" exists on self.
    NSString *key = SQSelfKeyPath(isFinished);
 
    // Verifies "isFinished" exists on instances of NSOperation.
    NSString *key = SQTypedKeyPath(NSOperation, isFinished);
 */
#define SQKeyPath(object, keyPath) ({ if (NO) { (void)((object).keyPath); } @#keyPath; })

#define SQSelfKeyPath(keyPath) SQKeyPath(self, keyPath)
#define SQTypedKeyPath(ObjectClass, keyPath) SQKeyPath(((ObjectClass *)nil), keyPath)
#define SQProtocolKeyPath(Protocol, keyPath) SQKeyPath(((id <Protocol>)nil), keyPath)