theorem Th31:
  for p being FinSequence of A
  for f being Function of A,D9 st q = f*p holds len q = len p
proof
  let p be FinSequence of A;
  let f be Function of A,D9;
  dom f = A & rng p c= A by FINSEQ_1:def 4,FUNCT_2:def 1;
  hence thesis by Th27;
end;
