code: def sheetNames(book=None):
      
      if book:
          if not book.lower() in [x.lower() for x in bookNames()]:
              return False
      else:
          book=activeBook()
      if not book:
          return False
      poBook=PyOrigin.WorksheetPages(book)
      if not len(poBook):
          return None
      return [x.GetName() for x in poBook.Layers()]
docs: return sheet names of a book.
  
      Args:
          book (str, optional): If a book is given, pull names from
              that book. Otherwise, try the active one
  
      Returns:
          list of sheet names (typical case).
          None if book has no sheets.
          False if book doesn't exlist.