Adds a node to the ProbModelXML.

def _add_variable(self, variable):
        """
        Adds a node to the ProbModelXML.
        """
        # TODO: Add feature for accepting additional properties of states.
        variable_data = self.data['probnet']['Variables'][variable]
        variable_element = etree.SubElement(self.variables, 'Variable', attrib={'name': variable,
                                                                                'type': variable_data['type'],
                                                                                'role': variable_data['role']})

        try:
            etree.SubElement(variable_element, 'Comment').text = variable_data['Comment']
        except KeyError:
            pass
        try:
            etree.SubElement(variable_element, 'Coordinates', variable_data['Coordinates'])
        except KeyError:
            pass

        try:
            for key, value in sorted(variable_data['AdditionalProperties'].items()):
                etree.SubElement(variable_element, 'Property', attrib={'name': key, 'value': value})
        except KeyError:
            etree.SubElement(variable_element, 'AdditionalProperties')
        states = etree.SubElement(variable_element, 'States')
        for s in sorted(variable_data['States']):
            state = etree.SubElement(states, 'State', attrib={'name': s})
            try:
                self._add_additional_properties(state, variable_data['States'][s]['AdditionalProperties'])
            except KeyError:
                etree.SubElement(state, 'AdditionalProperties')