Units map helper.  Each argument is treated as a unit
    descriptor--a tuple where the first element specifies a numerical
    value, and the second element consists of a sequence of names for
    that value.  Returns a dictionary where each name is mapped to the
    numerical value, and the numerical value is mapped to the first
    name.

def _make_units(*units):
    """
    Units map helper.  Each argument is treated as a unit
    descriptor--a tuple where the first element specifies a numerical
    value, and the second element consists of a sequence of names for
    that value.  Returns a dictionary where each name is mapped to the
    numerical value, and the numerical value is mapped to the first
    name.
    """

    units_map = {}
    for value, names in units:
        units_map[value] = names[0]
        for name in names:
            units_map[name] = value

    return units_map