
theorem Th20:
  for S being satisfying_Nat MusicStruct
  for n being non zero Nat holds n is Element of S
  proof
    let S be satisfying_Nat MusicStruct;
    let n be non zero Nat;
A1: NATPLUS c= the carrier of S by Def12a;
    n in NATPLUS by NAT_LAT:def 6;
    hence thesis by A1;
  end;
