reserve a,b,c,h for Integer;
reserve k,m,n for Nat;
reserve i,j,z for Integer;
reserve p for Prime;

theorem Th20:
  for a,b being object, f being FinSequence holds
  1 in dom (<*a,b*>^f) & 2 in dom (<*a,b*>^f)
  proof
    let a,b be object, f be FinSequence;
    set g = <*a,b*>;
A1: dom g = {1,2} by FINSEQ_1:92;
A2: 1 in {1,2} & 2 in {1,2} by TARSKI:def 2;
    dom g c= dom(g^f) by FINSEQ_1:26;
    hence thesis by A1,A2;
  end;
