Before revision:  We argue that the stall force generated by a group of filaments or motors is additive, i.e. , the stall force of N  filaments (motors) is N times the stall force of one filament (motor), when the system is in equilibrium at stall.
After revision:  We argue that the stall force generated by a group of filaments or motors is additive, that is , the stall force of N filaments (motors) is N times the stall force of one filament (motor), when the system is in equilibrium at stall.