ftp.nice.ch/pub/next/audio/apps/Tuner.NI.b.tar.gz#/Tuner/Frequency.h

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#import <soundkit/soundkit.h>

#define FREQUENCY_ERROR (-1.0)
#define FREQUENCY_UNSTABLE (0.0)

@interface Frequency : Object { }

- (double) computeFrequency:sample;

- (double) toleranceAtFrequency:(double) freq;

@end

These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.