Sets the default value of the given feature, overriding any previous default.
        feature: the name of the feature
        value: the default value to assign

def set_default (feature, value):
    """ Sets the default value of the given feature, overriding any previous default.
        feature: the name of the feature
        value: the default value to assign
    """
    f = __all_features[feature]
    bad_attribute = None

    if f.free:
        bad_attribute = "free"
    elif f.optional:
        bad_attribute = "optional"

    if bad_attribute:
        raise InvalidValue ("%s property %s cannot have a default" % (bad_attribute, f.name))

    if value not in f.values:
        raise InvalidValue ("The specified default value, '%s' is invalid.\n" % value + "allowed values are: %s" % f.values)

    f.set_default(value)