float exp_decay (float T_const, float t)
  {
    // the argument might not make mathematical sense (whatever.)
    assert(!isnan(T_const));
    assert(!isnan(t));
    if (T_const <= 0.001) {
      return 0.0;
    } else {
      float r = exp(- t / T_const);
      assert(!isnan(r));
      return r;
    }
  }