input: Write a python function to create a Book object and return its string representation.
  assert create_book("1984", "George Orwell", 1949, "1234567890") == "1984 by George Orwell published in 1949. ISBN: 1234567890"
  assert create_book("To Kill a Mockingbird", "Harper Lee", 1960, "0987654321") == "To Kill a Mockingbird by Harper Lee published in 1960. ISBN: 0987654321"
  assert create_book("Pride and Prejudice", "Jane Austen", 1813, "1122334455") == "Pride and Prejudice by Jane Austen published in 1813. ISBN: 1122334455"
output: def create_book(title, author, year, isbn):
      class Book:
          def __init__(self, title, author, year, isbn):
              self.title = title
              self.author = author
              self.year = year
              self.isbn = isbn
  
          def __str__(self):
              return f"{self.title} by {self.author} published in {self.year}. ISBN: {self.isbn}"
      
      book = Book(title, author, year, isbn)
      return str(book)