theorem Th16:
  S is Thue-system of E & s -->. t, S implies t -->. s, S
proof
  assume S is Thue-system of E & s -->. t, S;
  then S = S~ & [s, t] in S by RELAT_2:13;
  then [t, s] in S by RELAT_1:def 7;
  hence thesis;
end;
