def _extend_support(self, x, f, alt):
    """
    """
    # We need to do a series of broadcasts for the tf.where.
    scale = self.scale + tf.zeros_like(self.concentration)
    is_invalid = x < scale
    scale = scale + tf.zeros_like(x)
    x = x + tf.zeros_like(scale)
    # We need to do this to ensure gradients are sound.
    y = f(tf.where(is_invalid, scale, x))
    if alt == 0.:
      alt = tf.zeros_like(y)
    elif alt == 1.:
      alt = tf.ones_like(y)
    else:
      alt = tf.fill(
          dims=tf.shape(input=y),
          value=dtype_util.as_numpy_dtype(self.dtype)(alt))
    return tf.where(is_invalid, alt, y)