private int getLowIndex (double t) {
	if (this.nt == 0 || t < this.t[0])
		return -1;
	if (t >= this.t [this.nt-1])   /* Special case that often occurs in practice. */
		return this.nt-1;
	//Melder_assert (this.nt != 1);   /* May fail if t or my t [1] is NaN. */
	/* Start binary search. */
	int left = 0, right = this.nt-1;
	while (left < right - 1) {
		int mid = (left + right) / 2;
		if (t >= this.t [mid]) left = mid; else right = mid;
	}
	//Melder_assert (right == left + 1);
	return left;
    }