reserve X for set, x,y,z for object,
  k,l,n for Nat,
  r for Real;
reserve i,i0,i1,i2,i3,i4,i5,i8,i9,j for Integer;

theorem Th5:
  i2 <= i1 implies i1 - i2 in NAT
proof
  assume i2 <= i1;
  then i2 - i2 <= i1 - i2 by XREAL_1:9;
  hence thesis by Th3;
end;
