Require either of two arguments using argparse

Given:

import argparse

pa = argparse.ArgumentParser()
pa.add_argument('--foo')
pa.add_argument('--bar')

print pa.parse_args('--foo 1'.split())

how do I

• make at least one of “foo, bar” mandatory: --foo x, --bar y and --foo x --bar y are fine
• make at most one of “foo, bar” mandatory: --foo x or --bar y are fine, --foo x --bar y is not