Re: how does the query below work

[Laura]

The dictionary Id!PrevID is a unary. Applying the Converge iterator to a unary f derives a function f/ (f Over). When you evaluate f/[x], the unary f gets applied successively until either (a) the result stops changing (convergence) or (b) the result matches x i.e. comes full circle.

The white paper on iteration has some examples of iterating lists and dictionaries as finite-state machines.

All this assumes your ID => previous ID paths actually terminate! (You can use the Converge iterator to start an open loop!)