  The Steiner tree problems are well-known NP-hard problems that have diverse
applications. Duin et al. (2004) have intuitively proposed the widely-used
transformation from the classical group Steiner tree problem to the classical
Steiner tree problem in graphs. This transformation has not been rigorously
proven so far. Specifically, the large M value that is used in this
transformation has not been specified. In this paper, we address this issue by
rigorously prove this transformation for a specific large M value.
