reserve k,n for Nat,
  x,y,z,y1,y2 for object,X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for XFinSequence;
reserve D for set;
reserve i for Nat;

theorem
  for D being set, n being Nat, r being set st r in D holds
  n-->r is XFinSequence of D;
