  A corner is a triple of points in $\Bbb{Z}^2$ of the form
$(x,y),(x+d,y),(x,y+d)$ where $d\neq 0$. One can think of them as being
2D-analogues to 3-term arithmetic progressions.
  In this short note, we extend ideas of Green-Wolf from this latter setting to
the former, achieving slightly better constructions of corner-free sets.
