theorem Th2:
  (a,b // A or b,a // A) & a in A implies b in A
proof
  assume that
A1: a,b // A or b,a // A and
A2: a in A;
  a,b // A & A is being_line by A1,AFF_1:26,34;
  hence thesis by A2,AFF_1:23;
end;
