// add constant to each element
static double[] doubleAdd(double[] l, double a) {
  if (a == 0) ret l;
  int n = l(l);
  double[] x = new[n];
  for i to n:
    x[i] = l[i]+a;
  ret x;
}

// add element-wise
static double[] doubleAdd(double[] a, double[] b) {
  if (a == null) ret b;
  if (b == null) ret a;
  int n = l(a);
  assertEquals(l(b), n);
  double[] x = new[n];
  for i to n:
    x[i] = a[i]+b[i];
  ret x;
}