title: No visible interface error
body: <p>I have an error in the implementation file for my model which I have commented out. What can I do to fix this problem?</p>
  
  <p>Thanks in advance.</p>
  
  <pre><code>#import "CalculatorBrain.h"
  
  @interface CalculatorBrain()
  @property (nonatomic, strong) NSMutableSet *operandStack;
  @end
  
  @implementation CalculatorBrain
  
  @synthesize operandStack = _operandStack;
  
  - (NSMutableArray *)operandStack
  {
      if (!_operandStack) {
          _operandStack = [[NSMutableArray alloc] init];
      }
      return _operandStack;
  }
  
  -(void)pushOperand:(double)operand
  {
      NSNumber *operandObject = [NSNumber numberWithDouble:operand];
      [self.operandStack addObject:operandObject];
  }
  
  - (double)popOperand
  {
      NSNumber *operandObject = [self.operandStack lastObject]; // No visible interface for 'NSMutableSet' declares the selector 'lastObject'
      if(operandObject)   [self.operandStack removeLastObject]; // No visible interface for 'NSMutableSet' declares the selector 'removeLastObject'
      return [operandObject doubleValue];
  }
  
  - (double)performOperation:(NSString *)operation
  {
      double result = 0;
  
      if([operation isEqualToString:@"+"]) {
          result = [self popOperand] + [self popOperand];
      } else if ([@"*" isEqualToString:operation]) {
          result = [self popOperand] * [self popOperand];
      } else if ([operation isEqualToString:@"-"]) {
          double subtrahend = [self popOperand];
          result = [self popOperand] - subtrahend;
      } else if ([operation isEqualToString:@"/"]) {
          double divisor = [self popOperand];
          if (divisor)result = [self popOperand] / divisor;
      }
  
      [self pushOperand:result];
  
      return result;
  
  }
  
  
  @end
  </code></pre>