reserve i,j,k,l for natural Number;
reserve A for set, a,b,x,x1,x2,x3 for object;
reserve D,D9,E for non empty set;
reserve d,d1,d2,d3 for Element of D;
reserve d9,d19,d29,d39 for Element of D9;
reserve p,q,r for FinSequence;
reserve s for Element of D*;

theorem Th90:
  for D be set, z being FinSequence of D holds
  z is Element of (len z)-tuples_on D
proof
  let D be set, z be FinSequence of D;
  z is Element of D* by FINSEQ_1:def 11;
  then z in { z9 where z9 is Element of D*: len z9 = len z};
  hence thesis;
end;
