theorem
  not {A,B,C} is linear implies Plane(A,B,C) = Plane(B,C,A)
proof
  assume
A1: not {A,B,C} is linear;
  then
A2: not {B,C,A} is linear by ENUMSET1:59;
  thus Plane(A,B,C) = Plane(B,A,C) by A1,Th30
    .= Plane(B,C,A) by A2,Th29;
end;
