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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 compose_tree_path(tree, issn=False):
 """ Compose absolute path for given `tree`. Args: pub (obj):
 :class:`.Tree` instance. issn (bool, default False):
 Compose URL using ISSN. Returns: str: Absolute path of the tree, without server's address and protocol. """

    if issn:
        return join(
            "/",
            ISSN_DOWNLOAD_KEY,
            basename(tree.issn)
        )

    return join(
        "/",
        PATH_DOWNLOAD_KEY,
        quote_plus(tree.path).replace("%2F", "/"),
    )