theorem Th33:
  parallelogram a,b,c,d implies parallelogram a,c,b,d &
  parallelogram c,d,a,b & parallelogram b,a,d,c & parallelogram c,a,d,b &
  parallelogram d,b,c,a & parallelogram b,d,a,c & parallelogram d,c,b,a
proof
  assume
A1: parallelogram a,b,c,d;
  then parallelogram c,d,a,b by Th31;
  then
A2: parallelogram c,a,d,b by Th30;
  parallelogram b,a,d,c by A1,Th32;
  hence thesis by A1,A2,Th30,Th31;
end;
