code: public Direction direction() {
          if (!Op.isFinite(slope) || Op.isEq(slope, 0.0)) {
              return Direction.Zero;
          }
          if (Op.isGt(slope, 0.0)) {
              return Direction.Positive;
          }
          return Direction.Negative;
      }
docs: Returns the direction of the sigmoid
  
  @return the direction of the sigmoid