def _add_conversion_and_inverse(conv_dict, value, unit1, unit2):
    """Add a specific conversion and it's inverse.

    This function is mainly here to ensure that we don't forget to add
    the inverse conversions.

    Parameters
    ----------
    conv_dict : dict
        This is where we store the conversion.
    value : float
        The conversion factor to add
    unit1 : string
        The unit we are converting from.
    unit2 : string
        The unit we are converting to.

    Returns
    -------
    None, but updates the given `conv_dict`.

    """
    conv_dict[unit1, unit2] = value
    conv_dict[unit2, unit1] = 1.0 / conv_dict[unit1, unit2]
