Example of good assumptions 
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- "Material" and "Item" represent the same concept
- A collection is owned by exactly one branch
- A material belongs to exact one collection
- A collection can only contain one type of material
- Any kind of material should has an language attribute
- Each branch has a unique name
- Two collections at different branches may use same name
- A user can borrow as many items as she wishes 
- Videos and CDs can also in more than one languages
- Type (age group) of a user are defined by their age and 
  there is no overlapping between types. 
- We only store information about books, cds and videos 
  (not other kinds of material)
- A user can return an items to any library branch, we 
  need to keep tract this information
- A user may not return an item for ever

Examples of "not-so-good" assumptions
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- The database only store date of birth, let the applicaiton 
  to figure out user type and associate fine-rate 
  [not an assumption about the domain, but how you build your information system] 
- Library items are organized hierrachecally with item concept 
  as parent and book, cd and video concepts as children 
  [not an assumption about the domain, but a modeling technique used in a design decision] 
- Each item also has a loan period 
  [may be redundant since each user type already has an associated loan period] 
- A book can only be in one language 
  [this contradicts to what is clearly stated in the description]

Examples of contraints/information not captured 
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- What info need to store if an item is lost
- History of an item's condition
- Number of books each type of user can borrow
- Sequence of tasks a librarian performs to check out/in books
- Number of library branches
- Fine calculation