
theorem
  for l1,l2,l3 being Element of ProjectiveLines real_projective_plane st
  l1,l2,l3 are_concurrent holds l2,l1,l3 are_concurrent &
  l1,l3,l2 are_concurrent & l3,l2,l1 are_concurrent &
  l3,l2,l1 are_concurrent & l2,l3,l1 are_concurrent;
