@classmethod
def _json_param(cls, name, p, boxtype):
    """
        Return the parameter mode string and None if parameter unsupported.
        """
    mode = boxtype.mode(name)
    if mode != 'untyped' and not cls.supported(p):
        return None
    return {'name': name, 'label': boxtype.label(name), 'mode': mode, 'doc':
        cls.param_doc(p), 'value': cls.param_default(p), 'lims': cls.
        param_lims(p), 'step': cls.param_step(p)}
