We have two primitive sets in this model -- the component parts (PARTS) and the time periods (TIME).

From these two primitive sets, we create two derived sets.

The first derived set, USES, is a sparse set derived from the cross of the PARTS set on itself. We use this set to construct a data table, or input-output matrix (NEEDS), containing the parts usage data, which tells us how many of each of the other parts are required to produce a given part.  The set is sparse because not all parts are required in the production of certain other parts (e.g., chains are not required to produce a spoke).

The other derived set, PXT, is a dense set formed from the cross on the parts and time periods sets. We need this set because we will be concerned with the demand for each part in each period and the amount of each part to begin producing in each period.