- for "missing?" we often need to query database to check if an item exists. If predicates are truly independent, we will need to do the query again to return the item.
- "data flow" looks something like this (in Prismatic's Graph lib):
(def stats-graph
{:n (fnk [xs] (count xs))
:m (fnk [xs n] (/ (sum identity xs) n))
:m2 (fnk [xs n] (/ (sum #(* % %) xs) n))
:v (fnk [m m2] (- m2 (* m m)))})