theorem Th40:
  for A being being_line Subset of AS holds A // A
proof
  let A be being_line Subset of AS;
  consider a,b such that
A1: a<>b and
A2: A=Line(a,b) by Def3;
  a,b // a,b by Th1;
  hence thesis by A1,A2,Th36;
end;
