import shutil


def find_available_engine(engines=('pdftk', 'gs', 'pdfunite',
                                   'pdfjam', 'mutool')):
    """
    Build a list of available engines.


    """
    available = []
    for engine in engines:
        if shutil.which(engine) is not None:
            available.append(engine)
    if len(available) == 0:
        msg = f'No engine found among {engines}.'
        raise RuntimeError(msg)
    return available