private Bson newCall(Call existing, Operator newOperator) {
    return visit(Expressions.call(newOperator, existing.arguments()));
  }
see https://docs.mongodb.com/manual/reference/operator/query/not/
NOT operator is mongo is a little specific and can't be applied on all levels
$not: { $or ... } will not work and should be transformed to $nor
criteria/mongo/src/org/immutables/criteria/mongo/FindVisitor.java