
theorem Th28:
  for n, k being Element of NAT holds { PFArt (n,k) } is Element
  of SubstPoset (NAT, {k})
proof
  let n, k be Element of NAT;
  SubstitutionSet (NAT, {k}) = the carrier of SubstPoset (NAT, {k}) by
SUBSTLAT:def 4;
  hence thesis by HEYTING2:2;
end;
