// array-based stack: int x[MAXSIZE]; int toS;

AStack::AStack() { toS = -1; }

void AStack::clear() { toS = -1; }

bool AStack::isEmpty() const { return toS == -1; }

// 0, 1, 2   MAXSIZE = 3

void AStack::push(const int& el) { 
 if( toS < MAXSIZE-1) x[++toS] = el;
 else throw(stack_full_exception);
}

int AStack::pop() {
 if( toS >= 0) return x[toS--];
 else throw(stack_empty_exception);
}

int AStack::peek() { 
  int el = pop(); push(el); return el;
}