theorem Th74:
  x1, x2 ==>. y1, y2, TS implies ==>.-relation(TS) reduces [x1, x2 ], [y1, y2]
proof
  assume x1, x2 ==>. y1, y2, TS;
  then [[x1, x2], [y1, y2]] in ==>.-relation(TS) by Def4;
  hence thesis by REWRITE1:15;
end;
