cartan matrix :
   2  -2
  -1   2

root basis :
   2  -1
   2   0

coroot basis :
   1  -2
   0   1

positive roots :
[-1,0]
[0,2]
[1,2]
[2,2]

positive coroots :
[-2,1]
[-1,1]
[0,1]
[1,0]