def iter_plat_repos(repos, platforms):
    """ Generator that yield a SinglePlatformRespository for every repository,
    platform combination.

    Parameters
    ----------
    repos: list
        Containing Repository objects
    platforms: list
        Containing strings of valid platforms

    Yields
    ------
    SinglePlatformRepository:
        hatcher.core.v*.repository.SinglePlatformRespository instance

    """
    for repo in repos:
        for platform in platforms:
            yield repo.platform(platform)
