A state used to break up (complex) flow summaries into atomic flow steps. For a flow summary
propagatesFlow( SummaryComponentStack input, SummaryComponentStack output, boolean preservesValue )
the following states are used:
TSummaryNodeInputState(SummaryComponentStack s): this state represents that the components in
shave been read from the input.
TSummaryNodeOutputState(SummaryComponentStack s): this state represents that the components in
sremain to be written to the output.