Not null in Nemerle

gistfile1.n

public macro not_null(obj)
syntax ("not_null", obj)
{
    def var_name = $"$obj";
    <[ 
        when ($obj == null)
            throw System.ArgumentNullException($var_name);
        $obj;
    ]>
}

class Person
{
     public Name: string;
         
     public this(name: string)
     {
        Name = not_null name;
     }
}

// causes ArgumentNullException exception with argument name set to "name"!
def person = Person(null);