Draw the ERD for the library system.

The library system stores information about documents, which could be either a book or a journal.

1. The book information is: ISBN, name, subject area, availability status, book author.
2. The journal information is: ISBN, name, subject area, availability status, database Index, which is not allowed to have more than one journal or book with the same ISBN.
3. The student (StdNo, StdName) could borrow more than one document: either a book or a journal at specific date, and he/she must return the document at a specific return date. The document could be borrowed by more than one student at a different date when the book is available.
4. The employee (Empno, Ename) can only borrow a journal. However, he/she can borrow more than one journal for reading in the library at a specific date. The journal could be borrowed by more than one employee at a different time when the journal is available.

Note: dont forget to model if exists the following: Completeness Constraints, Disjointness Constraints, Subtype Discriminators.

please, explain the sentences in bold