def []=(index, value)
      raise TypeError, "instance is not mutable" unless mutable?
      self.class.check_cftype(value)
      CF.CFArraySetValueAtIndex(self, index, value)
      value
    end
def []=(index, value)
      raise TypeError, "instance is not mutable" unless mutable?
      self.class.check_cftype(value)
      CF.CFArraySetValueAtIndex(self, index, value)
      value
    end
Sets object at the index
 @param [Integer] index the 0 based index of the item to retrieve. Behaviour is undefined if it is not in the range 0..size
   It is legal to set the value at index n of a n item array - this is equivalent to appending the object

 @param [CF::Base] value the value to store
 @return [CF::Base] the store value