theorem Th31:
  L1,L2 are_coplane
  proof
    reconsider OO = |[0,0]|, Ox = |[1,0]|, Oy = |[0,1]| as Element of REAL 2
          by EUCLID:22;
    REAL 2 = plane(OO,Ox,Oy) by Th12;
    hence thesis by EUCLIDLP:def 12;
  end;
