UFC on ESPN 55 video: Ivana Petrovic submits gritty Liang Na with... rear-naked arm-triangle choke?

Ivana Petrovic pulled off an unusual submission Saturday on the UFC on ESPN 55 prelims.
While it took her several attempts throughout the bout, Petrovic (7-1 MMA, 1-1 UFC) eventually tapped Liang Na (19-8 MMA, 0-4 UFC) with what appeared to be a cross between an arm-triangle choke and a rear-naked choke. The official finish was categorized as an arm-triangle choke at 1:29 of Round 3.
Petrovic dominated much of the fight, outside of a brief armbar attempt by Na in Round 2. While Na survived much