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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sheetNames(book=None):
 """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. """

    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()]