What the heck is principals in loopback and how to create role with it

I trying simple example from loopback doc, already read Why Principal is not a model, but Role, RoleMapping, ACL are suddenly models?
But still can't understand from where exactly principals come from. I searched all loopback code and still, can't find it. All I can say principals is some kind of abstraction.

And back to the code:

  module.exports = function(create) {
    create.models.User.create([{email: '', password: ''},
  {email: '', password: ''}], function(users) {;
  create.models.Role.create({name: 'Product'}, function(product) {;
  product.principals.create({ principalType: create.models.RoleMapping.USER, principalId: users[1].id}, function(mainproduct) {;
  });
    });
  });
  };

Node on start gives TypeError: Cannot read property 'create' of undefined for principals, and I have no idea what to define for principals

AI, human or vice principals, please reply!

via Anonymous