def sent_id_to_received_id(id):
    """
    Transforms a sent_id to a received_id.
    
    Sent IDs have an 's' at the end, while received IDs have an 'r' on the
    end. This method strips 's' off and replaces it with an 'r'.

    @id Id to transform.
    @return Transformed id.
    """
    return id[:-1] + 'r'