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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 subdomains_init(blockstack_opts, working_dir, atlas_state):
 """ Set up subdomain state Returns a SubdomainIndex object that has been successfully connected to Atlas """

    if not is_subdomains_enabled(blockstack_opts):
        return None

    subdomain_state = SubdomainIndex(blockstack_opts['subdomaindb_path'], blockstack_opts=blockstack_opts)
    atlas_node_add_callback(atlas_state, 'store_zonefile', subdomain_state.enqueue_zonefile)

    return subdomain_state