code: public Vector2d transformPosition(Vector2d v) {
          v.set(m00 * v.x + m10 * v.y + m20,
                m01 * v.x + m11 * v.y + m21);
          return v;
      }
docs: Transform/multiply the given 2D-vector, as if it was a 3D-vector with z=1, by
  this matrix and store the result in that vector.
  <p>
  The given 2D-vector is treated as a 3D-vector with its z-component being 1.0, so it
  will represent a position/location in 2D-space rather than a direction.
  <p>
  In order to store the result in another vector, use {@link #transformPosition(Vector2dc, Vector2d)}.
  
  @see #transformPosition(Vector2dc, Vector2d)
  @see #transform(Vector3d)
  
  @param v
  the vector to transform and to hold the final result
  @return v