reserve n for Nat,
  k for Integer;
reserve p for polyhedron,
  k for Integer,
  n for Nat;

theorem Th28:
  num-polytopes(p,-1) = 1
proof
  reconsider minusone = -1 as Integer;
  minusone-polytopes(p) = {{}} by Def5;
  hence thesis by CARD_1:30;
end;
