theorem Th2:
  a _|_ b implies b _|_ a
proof
  set 0V = 0.S;
  assume a _|_ b;
  then
A1: a _|_ 0V+b by RLVECT_1:4;
  0V _|_ b+a by Th1;
  then b _|_ a+0V by A1,Def1;
  hence thesis by RLVECT_1:4;
end;
