reserve x for set;
reserve k, l for Nat;
reserve p, q for FinSequence;
reserve R for Relation;
reserve p, q for RedSequence of R;
reserve E for set;
reserve s, t for XFinSequence;
reserve p, q for XFinSequence-yielding FinSequence;

theorem Th5:
  len (s ^+ p) = len p & len(p +^ s) = len p
proof
  dom (s ^+ p) = dom p & dom (p +^ s) = dom p by Def2,Def3;
  hence thesis by FINSEQ_3:29;
end;
