reserve n for Nat,
  k for Integer;

theorem Th21:
  for X,Y being set holds [:X,Y:] --> 1.Z_2 is incidence-matrix of X,Y
proof
  let X,Y be set;
  set f = [:X,Y:] --> 1.Z_2;
  rng f c= {1.Z_2} & {1.Z_2} c= {0.Z_2,1.Z_2} by FUNCOP_1:13,ZFMISC_1:7;
  then dom f = [:X,Y:] & rng f c= {0.Z_2,1.Z_2};
  hence thesis by FUNCT_2:def 2;
end;
