$1)$ The system should include user interfaces that support creation, modification, and deletion of: a$)$ Customers b$)$ Suppliers c$)$ Products $2)$ The system should include user interfaces that support creation and modification of: a$)$ Sales orders $$ customers must be able to have more than one order b$)$ Purchase orders $$ suppliers must be able to have more than purchase one order $3)$ The system should include functionality that: a$)$ Calculates stock levels for different products b$)$ Prevents users from selling products that are not in stock