In one embodiment of the present invention, a media service system provides at least one transaction configuration option that is enabled to be selected by a user. More specifically, the present invention comprises two or more electrostatic reflectors positioned coaxially with respect to one another such that ions generated by an ion source can be reflected back and forth between them. A plurality of arms are coupled to the mounting bracket and each include a distal end to be positioned at an elevation vertically above an elevation of the belt of the treadmill. A forward assumption propagation method generates assumptions to characterize a set of interesting states for a property being evaluated at one or more evaluation stages.