theorem
  not {A,B,C} is linear implies Plane(A,B,C) = Plane(C,B,A)
proof
  assume
A1: not {A,B,C} is linear;
  then
A2: not {C,B,A} is linear by ENUMSET1:60;
  thus Plane(A,B,C) = Plane(C,A,B) by A1,Th32
    .= Plane(C,B,A) by A2,Th29;
end;
