returns the lowercased string of the connective used in the given Conano unit.

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))