// This method removes all items which matches the predicate.
        // The complexity is O(n). 
        public int RemoveAll(Predicate<T> match) {
            if( match == null) { 
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match); 
            }
            Contract.Ensures(Contract.Result<int>() >= 0); 
            Contract.Ensures(Contract.Result<int>() <= Contract.OldValue(Count));
            Contract.EndContractBlock();
 
            int freeIndex = 0;   // the first free slot in items array 
 
            // Find the first item which needs to be removed. 
            while( freeIndex < _size && !match(_items[freeIndex])) freeIndex++; 
            if( freeIndex >= _size) return 0;
 
            int current = freeIndex + 1;
            while( current < _size) {
                // Find the first item which needs to be kept.
                while( current < _size && match(_items[current])) current++; 
 
                if( current < _size) { 
                    // copy item to the free slot. 
                    _items[freeIndex++] = _items[current++];
                } 
            }
 
            Array.Clear(_items, freeIndex, _size - freeIndex);
            int result = _size - freeIndex; 
            _size = freeIndex;
            _version++; 
            return result; 
        }