add comments and assertions.

  const static double N_div_beta = parameters.get_sp_time_intervals() / beta;

  int sign = 1;

  // Map tau to [0, beta).

  if (tau < 0) {

    tau += beta;

    sign = -1;

  }

  assert(0 <= tau && tau < beta);

  const double scaled_tau = tau * N_div_beta;

  const int t_ind = scaled_tau;

  // Find interpolation index of on the left of tau.

  const int t_ind = static_cast<int>(scaled_tau);

#ifndef NDEBUG

  const double* positive_times =