Returns join(root_path, subpath) where subpath in valid_subpath ane
    exists(subpath)

def existing_subpath(root_path, valid_subpaths, tiebreaker='first',
                     verbose=VERYVERBOSE):
    """
    Returns join(root_path, subpath) where subpath in valid_subpath ane
    exists(subpath)
    """
    # Find the oxford_style groundtruth directory
    for subpath in valid_subpaths:
        path  = join(root_path, subpath)
        if checkpath(path, verbose=verbose):
            if tiebreaker == 'first':
                return path
    raise AssertionError('none of the following subpaths exist: %r' %
                         (valid_subpaths,))