def fg_validator(passed_kwargs, merged_kwargs):
    """ fg is not valid with basically every other option """
    invalid = []
    msg = """_fg is invalid with nearly every other option, see warning and workaround here:

    https://amoffat.github.io/sh/sections/special_arguments.html#fg"""
    whitelist = set(('env', 'fg', 'cwd'))
    offending = set(passed_kwargs.keys()) - whitelist
    if 'fg' in passed_kwargs and passed_kwargs['fg'] and offending:
        invalid.append(('fg', msg))
    return invalid
