docs: @javax.annotation.Nonnull
    public static double[] op(@javax.annotation.Nonnull final double[] a, @javax.annotation.Nonnull final double[] b, @javax.annotation.Nonnull final DoubleBinaryOperator fn) {
      assert a.length == b.length;
      @javax.annotation.Nonnull final double[] c = new double[a.length];
      for (int j = 0; j < a.length; j++) {
        c[j] = fn.applyAsDouble(a[j], b[j]);
      }
      return c;
    }