// ---- @interface ----
  @interface Fraction : NSObject {  // inherit from NSObject
      int numerator;
      int denominator;
  }
  - (void)print;
  - (void)setNumerator:(int)n;
  - (void)setDenominator:(int)d;
  @end

  // ---- @implementation ----
  @implementation Fraction   // you can optionally specify :NSObject after Fraction
  - (void)print { NSLog(@"%i/%i", numerator, denominator); }
  - (void)setNumerator:(int)n { numerator = n; }
  - (void)setDenominator:(int)d { denominator = d; }
  @end