Errors using Metamodel::ConcreteRoleHOW.new_type

Question

There does not seem to be a way to use new_type in MetamodelConcreteRoleHOW, which as its name implies should be used to create new instances of a Role. The main problem is when you try to mix-in new roles, as implied by the signature ( method new_type(:@roles, :$name = '<anon>', :$ver, :$auth, :$repr, :$api)):

my $a = Metamodel::ConcreteRoleHOW.new_type(name => "Bar", roles => [Iterable]);
$a.^compose;
say $a.^roles;
# Error: «Cannot iterate object with P6opaque representation (Array)␤

Using another Positional, a list, yields a different error:

my $a = Metamodel::ConcreteRoleHOW.new_type(name => "Bar", roles => (Iterable));
$a.^compose;
say $a.^roles
# « Cannot iterate over a Iterable type object␤»

Beats me what kind of positional could I use there. To be sure, this is implemented in NQP, so maybe I should be defining an NQP array. But I really have no idea. Any help will be appreciated.

Edit. First, the error happens when you call compose. Second, you effectively have to use an NQP array as Raiph says

use nqp;
my $roles := nqp::list(Iterable);
my $a = Metamodel::ConcreteRoleHOW.new_type(name => "Bar", roles => $roles);
$a.^compose;
say $a.^is_composed();
say $a.^roles # OUTPUT: «1␤(Mu)␤»

compose works now, and it is actually composed, but the roles composed still show only the ur-role, Mu, not Iterable which is the one that should have been added to it. Any idea?