Steiner Tree
Tree interconnecting a set of points (demand points, D) and some other points (Steiner points, S)
If S is empty, Steiner Minimum Tree (SMT) equivalent to Minimum Spanning Tree (MST)
Finding SMT is NP-complete
Many excellent heuristics
Underlying Grid Graph defined by intersection of horizontal and vertical lines through demand points, leads to Rectilinear Steiner Tree (RST) and RSMT problems
Can compute MST and modify to approximate RMST