reserve k,n for Nat,
  x,y,z,y1,y2 for object,X,Y for set,
  f,g for Function;

theorem Th0: ::: CHORD:1 moved eventually from there -> go to INT_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 INT_1:3;
   hence n-1 is Nat;
   thus thesis by A1;
end;
