copy method
Implementation
CountTable<K> copy({bool Function(K key, int count)? filter}) {
var copy = CountTable<K>();
if (filter != null) {
for (var e in _table.entries) {
var key = e.key;
var value = e.value;
if (filter(key, value.count)) {
copy._table[key] = value.copy();
}
}
} else {
for (var e in _table.entries) {
copy._table[e.key] = e.value.copy();
}
}
return copy;
}