Trivial reformatting change.
Remove use of option workflow in the calculation of a node's combined filter - use precondition checking in 'Node.WithFilter' to screen out insane filters instead. This sets up reuse of 'walkTree' in a fortcoming commit (and makes it easier for clients to spot insane filters)...
WIP commit: now building up lists of nodes with ordinary filters and nodes with filters for tagged inputs, rather than lists of the filters themselves. This will allow downstream code to use the nodes when building tagged filter inputs.