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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 get_connective(docgraph, unit_id):
 """ returns the lowercased string of the connective used in the given Conano unit. """

    unit_index, _unit_type = unit_id.split(':')
    connective_id = unit_index+':connective'
    return ' '.join(docgraph.get_token(tok_id).lower()
                    for tok_id in get_span(docgraph, connective_id))