theorem Th19:
  <*x*> ==>. y implies
  <*>the carrier of s ==>. y/"x & <*>the carrier of s ==>. x\y
proof
  assume
A1: <*x*> ==>. y;
A2: e(s)^<*x*> = <*x*> by FINSEQ_1:34;
  <*x*>^e(s) = <*x*> by FINSEQ_1:34;
  hence thesis by A1,A2,Def18;
end;
