  Influence diagrams have recently been used to analyse the safety and fairness
properties of AI systems. A key building block for this analysis is a graphical
criterion for value of information (VoI). This paper establishes the first
complete graphical criterion for VoI in influence diagrams with multiple
decisions. Along the way, we establish two important techniques for proving
properties of multi-decision influence diagrams: ID homomorphisms are
structure-preserving transformations of influence diagrams, while a Tree of
Systems is collection of paths that captures how information and control can
flow in an influence diagram.
