A classical approach of task planning is to build complex robot behaviours based on hierarchal structure (Nakamura, Hanafusa, & Yoshikawa, 1987; Mansard & Chaumette, 2007). In literatures, different ways to define hierarchy among the tasks have been explored. In Decre, Smits, Bruyninckx, and De Schutter (2009), the priority between the tasks is defined by weighting the influence of each task with respect to the others. In Escande, Mansard, and Wieber (2010) and Siciliano and Slotine (1991) a stack-of-task mechanism is defined, where the priority between the tasks is ensured by realizing each task in the null space left by tasks of higher priority.