reserve i,j,k,n for Nat;
reserve x,y,z for Tuple of n, BOOLEAN;

theorem Th17:
  i is_expressible_by n implies ChangeVal_1(i,n) <= 2 to_power(n)
  & ChangeVal_1(i,n) > 0
proof
  assume
A1: i is_expressible_by n;
A2: 2 to_power(n) > 0 by POWER:34;
  per cases;
  suppose
    i=0;
    hence thesis by A2,Def7;
  end;
  suppose
A3: i<>0;
    then ChangeVal_1(i,n) = i by Def7;
    hence thesis by A1,A3;
  end;
end;
