Я ☞ Methods ☞ at

It’s a method that lets you use a Reference by a type signature.

at : Reference structure substructure substructure

You need to provide a type siganture so compiler will try to find it within a structure:

(Scope `hv` at @Boolean) : Attribute (Boolean `P` Integer) Boolean
(Scope `hv` at @Boolean) : Attribute (Integer `P` Boolean) Boolean