reserve a,b,c for set;

theorem Th1:
  for p,q,r,s being FinSequence holds
  p^q^r^s=p^(q^r)^s & p^q^r^s=(p^q)^(r^s) & p^(q^r)^s=(p^q)^(r^s)
proof
  let p,q,r,s be FinSequence;
  p^q^r^s=(p^q)^(r^s) by FINSEQ_1:32;
  hence thesis by FINSEQ_1:32;
end;
