Helper function to add variable tag to the potential_tag

        Parameters
        ----------
        potential: dict
            Dictionary containing Potential data.
            For example: {'role': 'Utility',
                          'Variables': ['D0', 'D1', 'C0', 'C1'],
                          'type': 'Tree/ADD',
                          'UtilityVaribale': 'U1'}
        var: string
            New Element tag which needs to be added to the potential tag.
            For example: 'type'
        potential_tag: etree Element
            etree element which would contain potential tag
            For example: <Element Potentials at 0x7f315fc44b08>
                         <Element Branch at 0x7f315fc44c88>
                         <Element Branch at 0x7f315fc44d88>
                         <Element Subpotentials at 0x7f315fc44e48>

        Examples
        --------
        >>> writer = ProbModelXMLWriter(model)
        >>> writer._add_element(potential, 'State', parent_tag)

def _add_element(potential, var, potential_tag):
        """
        Helper function to add variable tag to the potential_tag

        Parameters
        ----------
        potential: dict
            Dictionary containing Potential data.
            For example: {'role': 'Utility',
                          'Variables': ['D0', 'D1', 'C0', 'C1'],
                          'type': 'Tree/ADD',
                          'UtilityVaribale': 'U1'}
        var: string
            New Element tag which needs to be added to the potential tag.
            For example: 'type'
        potential_tag: etree Element
            etree element which would contain potential tag
            For example: <Element Potentials at 0x7f315fc44b08>
                         <Element Branch at 0x7f315fc44c88>
                         <Element Branch at 0x7f315fc44d88>
                         <Element Subpotentials at 0x7f315fc44e48>

        Examples
        --------
        >>> writer = ProbModelXMLWriter(model)
        >>> writer._add_element(potential, 'State', parent_tag)
        """
        if var in potential:
            etree.SubElement(potential_tag, var).text = potential[var]