The search control parameter contains a program which determines what information is extracted from the database for a particular diagram. This program always starts with the database node pointed at by the current parameter. The program extracts the graph of components and composites stemming from this node. It maintains a pointer that determines which level in the graph is currently of interest - the active level. Each instruction in the program will be one of the following:

• m - moves the pointer to a higher level
• p - moves the pointer to a lower level
• u - extracts from the database all the composites of the nodes in the active level
• d - extracts from the database all the components of the nodes in the active level
• e - the program must always end with this instruction