def half_normal_like(x, tau): 
    r""" 
    half_normal_like(x, tau)

    Half-normal log-likelihood, a normal distribution with mean 0 and limited
    to the domain :math:`x \in [0, \Infty)`.

    .. math::
        f(x \mid \tau) = \sqrt{\frac{2\tau}{\pi}}\exp

\left\{ {\frac{-x^2 \tau}{2}}\right\} 

    :Parameters:
      x : float
        :math:`x \ge 0`
      tau : float
        :math:`\tau > 0`

    """
    # try:
    #     constrain(tau, lower=0)
    #     constrain(x, lower=0, allow_equal=True) 
    # except ZeroProbability:
    #     return -Inf
    return flib.hnormal(x, tau)