if (num == 0)
return *this;
- basic *copy = NULL;
+ basic *copy = nullptr;
for (size_t i=0; i<num; i++) {
const ex & o = op(i);
const ex & n = f(o);
if (!are_ex_trivially_equal(o, n)) {
- if (copy == NULL)
+ if (copy == nullptr)
copy = duplicate();
copy->let_op(i) = n;
}