theorem Th30:
  not {A,B,C} is linear implies Plane(A,B,C) = Plane(B,A,C)
proof
  assume
A1: not {A,B,C} is linear;
  then not {B,A,C} is linear by ENUMSET1:58;
  then {B,A,C} on Plane(B,A,C) by Def20;
  then {A,B,C} on Plane(B,A,C) by ENUMSET1:58;
  hence thesis by A1,Def20;
end;
