def _make_rv_params_variable(rv, **params):
    """
    Wraps the random variable rv, allowing it
    to accept time-dependent parameters.
    """
    if any(callable(x) for x in params.values()):
        return lambda t: rv(
            **{k: (x(t) if callable(x) else x)
               for k, x in params.items()})
    else:
        return rv(**params)