code: public static void subQuad (double[] coef, double t0, boolean left) {
          if (left) {
              coef[2] = (1 - t0) * coef[0] + t0 * coef[2];
              coef[3] = (1 - t0) * coef[1] + t0 * coef[3];
          } else {
              coef[2] = (1 - t0) * coef[2] + t0 * coef[4];
              coef[3] = (1 - t0) * coef[3] + t0 * coef[5];
          }
      }
docs: t0 - ?