def project_vintages_operational_in_period(
        project_vintage_set, operational_periods_by_project_vintage_set,
        period):
    """
    :param project_vintage_set: possible project-vintages when capacity
        could be built
    :param operational_periods_by_project_vintage_set: the periods when
        project capacity of a particular vintage could be operational
    :param period: the period we're in
    :return: all vintages that could be operational in a period

    Get the project vintages that COULD be operational in each period.
    """
    project_vintages = list()
    for (prj, v) in project_vintage_set:
        if period in operational_periods_by_project_vintage_set[prj, v]:
            project_vintages.append((prj, v))
        else:
            pass
    return project_vintages