Adds a typed child object to the structure object.

        @param child: Child object to be added.

def add(self, child):
        """
        Adds a typed child object to the structure object.

        @param child: Child object to be added.
        """

        if isinstance(child, Assign):
            self.add_assign(child)
        else:
            raise ModelError('Unsupported child element')