Affix works on class instances similar to how Append works on Association objects. Single new elements are specified in the form of key->value. New values replace older ones if their keys are the same.
Delayed values can be specified using key:>value.
Multiple elements to affix to a class instance can be specified in the form {key1->value1,key2->value2,…}.
Affix[element][instance] is equivalent to Affix[instance, element].