  We present a method for hierarchically generating sound workflow nets by
substitution of nets with multiple inputs and outputs. We show that this method
is correct and generalizes the class of nets generated by other hierarchical
approaches. The method involves a new notion of soundness which is preserved by
the generalized type of substitution that is presented in this paper. We show
that this notion is better suited than *-soundness for use with the presented
type of generalized substitution, since {*}-soundness is not preserved by it.
It is moreover shown that it is in some sense the optimal notion of soundness
for the purpose of generating sound nets by the presented type of substitution.
