text: /**
   * Author: Simon Lindholm
   * Date: 2015-04-22
   * License: CC0
   * Source: me
   * Description: Faster-than-naive convolution of two sequences: $c[x] = \sum a[i]b[x-i]$.
     Uses the identity $(aX+b)(cX+d) = acX^2 + bd + ((a+c)(b+d)-ac-bd)X$.
     Doesn't handle sequences of very different length well.
     See also FFT, under the Numerical chapter.
   * Time: O(N^{1.6})
   * Status: tested
   */
  