mapM -containers -package
Map each element of a structure to a monadic action, evaluate these actions from left to right, and ignore the results.
The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it just Just b, then b is included in the result list.
Graph construction monad; handles passing both the NodeMap and the Graph.