Associated with SLOT are attributes that each instance will inherit.
Each slot has a domain and range.
Range is split into two parts one the class of the elements and the other is a constraint which is a logical expression if absent it is taken to be true.
If there is a value for default then it must be passed on unless an instance has its own value.
The to-compute attribute involves a procedure to compute its value. E.g. in Position where we use the dot notation to assign values to the slot of a frame.
Transfers through lists other slots from which values can be derived
