Model fragments has functions from instances to quantities. These functions are called quantity-function's. Has-quantity-function is used to name the relevant functions for an model fragment instance. It is typically defined as an instance slot on a model-fragment class.
Suporting the instances option, the relevant slots inherit down through subclasses of model-fragment.
In support of the class option, this allows an inverse slot (currently called QUANTITY-FUNCTION-OF) from functions to the model-fragments it is about.