Specman reflection: Generic method to copy list of any type

I would like to write a generic method that copies list of any type. My code:

copy_list(in_list : list of rf_type) : list of rf_type is {
    var out_list : list of rf_type;
    gen out_list keeping { it.size() == in_list.size(); };
    for each (elem) in in_list {
        out_list[index] = elem.unsafe();
    };
    result = out_list;
};

The method's call:

var list_a : list of uint;
gen list_a;
var list_b : list of uint;
list_b = copy_list(list_a);

The error:

   ERR_GEN_NO_GEN_TYPE: Type 'list of rf_type' is not generateable
(it is not a subtype of any_struct) at line ...
        gen out_list keeping { it.size() == in_list.size(); };

In addition, there is an error also when defining the method copy_list using list of untyped:

Error: 'list_a' is of type 'list of uint', while expecting
type 'list of untyped'.

Can you please help to write a generic method to copy list? Thank you for any help