'Append a value to the DiskSet (only if the value is not already
contained in this instance).
:param value: The value to append.
:return: True if the value was added. False if it existed and was not
added.'

def add(self, value):

    with self.lock:
        if self.__contains__(value):
            return False
        else:
            super(DiskSet, self).append(value)
            return True
