theorem Th29:
  not {A,B,C} is linear implies Plane(A,B,C) = Plane(A,C,B)
proof
  assume
A1: not {A,B,C} is linear;
  then not {A,C,B} is linear by ENUMSET1:57;
  then {A,C,B} on Plane(A,C,B) by Def20;
  then {A,B,C} on Plane(A,C,B) by ENUMSET1:57;
  hence thesis by A1,Def20;
end;
