  Kengo Matsumoto has introduced lambda-graph systems and strong shift
equivalence of lambda-graph systems [Doc.Math.4 (1999), 285-340]. We associate
to a subshift of a subshift a lambda-graph system. The strong shift equivalence
class of the associated lambda-graph system is an invariant of subsystem
equivalence. Wolfgang Krieger and Kengo Matsumoto have introduced the
lambda$-entropy of a lambda-graph system and have shown its invariance under
strong shift equivalence [Ergod.Th.&Dynam.Sys. 24 (2004) 1155 - 1172]. A
separation entropy of a subshift of a subshift is introduced as the
lambda-entropy of the associated lambda-graph system.
