ABC Ltd plans to computerise its sales ordering and stock control system. A feasibility study hasstrongly suggested that a relational database system be installed. The details of ABC's sales and stock control are as follows:

Customers send in orders for goods. Each order may contain requests for variable quantities of one or more products from ABC's range. ABC keeps a stock file showing for each product the product details and the preferred supplier, the quantity in stock, the reorder level and other details.

ABC delivers those goods that it has in stock in response to the customer order and an invoice is produced for the despatched items. Any items that were not in stock are placed on a back order list and these items are usually re-ordered from the preferred supplier. Occasionally items are ordered from alternative sources.

In response to the invoices that are sent out to ABC's customers, the customers send in payments. Sometimes a payment will be for one invoice, sometimes for part of an invoice and sometimes for several invoices and part-invoices.

Draw an entity-relationship model, stating any assumptions made. Make sure you remove all many-many relationships and replace them with new entity types and relationship types.