com.twitter.summingbird.planner.DagOptimizer
If the given Id can be replaced with a simpler expression, return Some(expr) else None.
If the given Id can be replaced with a simpler expression, return Some(expr) else None.
If it is convenient, you might write a partial function and then call .lift to get the correct Function type
(a.flatMap(f1) ++ a.flatMap(f2)) == a.flatMap { i => f1(i) ++ f2(i) }