theorem Th26:
  S c= T implies ==>.-relation(S) c= ==>.-relation(T)
proof
  assume
A1: S c= T;
 let x be object;
    assume
A2: x in ==>.-relation(S);
    then consider s, t being object such that
A3: x = [s, t] and
A4: s in E^omega & t in E^omega by RELSET_1:2;
    reconsider s, t as Element of E^omega by A4;
    s ==>. t, S by A2,A3,Def6;
    then s ==>. t, T by A1,Th19;
    hence thesis by A3,Def6;
end;
