sentences:  On 7 June 1871 Hristo "the bookbinder" Ivanov invited Nikola Obretenov to take part in the revolutionary struggle as a messenger between Bulgaria and BRCK in Bucharest.On 29 June 1871 Obretenov travelled to Bucharest to present his references to the chairman of BRCK, Lyuben Karavelov.After being approved by Karavelov, Nikola Obretenov along with Dimitar Gorov of Giurgiu developed an illegal channel for transferring mail, printed materials, and weapons between Bucharest–Giurgiu and Rousse.