reserve i, j, k, c, m, n for Nat,
  a, x, y, z, X, Y for set,
  D, E for non empty set,
  R for Relation,
  f, g for Function,
  p, q for FinSequence;

theorem Th17:
  arity {} = 0
proof
  not ex x being FinSequence st x in dom {};
  hence thesis by MARGREL1:def 25;
end;
