def id(left_padding=None, right_padding=None, offset=None):

    """Constructs an implicit-duration spec dictionary."""

    result = {}

    if left_padding is not None:
        result['left_padding'] = left_padding

    if right_padding is not None:
        result['right_padding'] = right_padding

    if offset is not None:
        result['offset'] = offset

    return result