I. Introduction
MULTIPLE STRUCTURE ALIGNMENT (MStA) is an essential task in structural bioinformatics, which is useful for protein structure classification, structure-based function prediction, and highlighting structurally conserved regions of functional significance. In addition, MStA can be used to gain deeper insights into the evolutionary history of proteins (see [6] and references therein). As a result, various MStA algorithms have been developed (see e.g. [3], [5], [6], [8], [10]).