  We find the equations that allow us to compute the position of the two
interior nodes (weighted Fermat-Torricelli points) w.r. to the weighted Steiner
problem for four points determining a tetrahedron in R^3. Furthermore, by
applying the solution w.r. to the weighted Steiner problem for a boundary
tetrahedron, we calculate the twist angle between the two weighted Steiner
planes formed by one edge and the line defined by the two weighted Fermat-
Torricelli points and a non-neighbouring edge and the line defined by the two
weighted Fermat-Torricelli points.
