def add(self, key):
        """Adds a key to this bloom filter.
        If the key already exists in this filter it will return True.
        Otherwise False.

        >>> b = ScalableBloomFilter(initial_capacity=100, error_rate=0.001, \
                                    mode=ScalableBloomFilter.SMALL_SET_GROWTH)
        >>> b.add("hello")
        False
        >>> b.add("hello")
        True

        """
        if key in self:
            return True
        if not self.filters:
            filter = BloomFilter(
                capacity=self.initial_capacity,
                error_rate=self.error_rate * (1.0 - self.ratio))
            self.filters.append(filter)
        else:
            filter = self.filters[-1]
            if filter.count >= filter.capacity:
                filter = BloomFilter(
                    capacity=filter.capacity * self.scale,
                    error_rate=filter.error_rate * self.ratio)
                self.filters.append(filter)
        filter.add(key, skip_check=True)
        return False
Adds a key to this bloom filter.
        If the key already exists in this filter it will return True.
        Otherwise False.

        >>> b = ScalableBloomFilter(initial_capacity=100, error_rate=0.001, \
                                    mode=ScalableBloomFilter.SMALL_SET_GROWTH)
        >>> b.add("hello")
        False
        >>> b.add("hello")
        True