theorem :: moved from CHORD:1
  for n being non zero Nat holds n-1 is Nat & 1 <= n
  proof
    let n be non zero Nat;
A1: 0+1 <= n by NAT_1:13;
    then 0+1-1 <= n-1 by XREAL_1:9;
    then n-1 in NAT by Th3;
    hence n-1 is Nat;
    thus thesis by A1;
end;
