theorem
  for N being invertible Matrix of 3,F_Real holds
  dom homography(N) = ProjectivePoints TOP-REAL 3
  proof
    let N be invertible Matrix of 3,F_Real;
    dom homography(N) = the carrier of ProjectiveSpace TOP-REAL 3
      by FUNCT_2:def 1;
    hence thesis by ANPROJ_1:23;
  end;
