'parallel : int, optional
When provided, run the tests in parallel on the specified
number of CPUs.  If parallel is negative, it will use the all
the cores on the machine.  Requires the ``pytest-xdist`` plugin.'

@keyword(0)
def parallel(self, parallel, kwargs):

    if (parallel != 0):
        try:
            from xdist import plugin
        except ImportError:
            raise SystemError(u'running    tests    in    parallel    requires    the    pytest-xdist    package')
        return [u'-n', six.text_type(parallel)]
    return []
