theorem Th35:
  [[x1, x2], [y1, y2]] in ==>.-relation(TS) implies ex v, w st v =
  y2 & x1, w -->. y1, TS & x2 = w^v
proof
  assume [[x1, x2], [y1, y2]] in ==>.-relation(TS);
  then x1, x2 ==>. y1, y2, TS by Def4;
  hence thesis;
end;
