def METHOD_NAME(cls, capability_string):
    """
    :returns Capability: a new instance from the given capability-string, or an
        exception if it is not well-formed.
    """
    cap = _tahoe_uri_from_string(capability_string)
    if isinstance(cap, UnknownURI):
        raise ValueError(
            "Invalid capability-string {}".format(capability_string)
        )
    return cls(
        uri=capability_string,
        tahoe_cap=cap,
    )