GLOBALTYPE struct_border_step Local-plan_undi structure for border_step. In this context, a border_step represents an gdtedge with an associated moving direction. Namely, each gdtedge e=(v1,v2) has associated exactly two border_steps: 'bs1' moving from v1 and 'bs2' moving from v2. Nodes v1 and v2 are called 'start' ('stop') of border_steps bs1(bs2) and bs2(bs1), respectively (don't confuse with the start and stop nodes of directed edges). bs1(bs2) is said to be the the 'opposite' border_step of bs2(bs1). Also, the right face that we see moving along gdtedge e from gdtnode v1(v2) will be called the 'face of border_step bs1 (bs2)'. Finally, gdtedge e is called 'gdtedge of border steps bs1 and bs2'.