Updated the contents of the current AttrTree with the
        contents of a second AttrTree.

def update(self, other):
        """
        Updated the contents of the current AttrTree with the
        contents of a second AttrTree.
        """
        if not isinstance(other, AttrTree):
            raise Exception('Can only update with another AttrTree type.')
        fixed_status = (self.fixed, other.fixed)
        (self.fixed, other.fixed) = (False, False)
        for identifier, element in other.items():
            if identifier not in self.data:
                self[identifier] = element
            else:
                self[identifier].update(element)
        (self.fixed, other.fixed) = fixed_status