public struct Map: Fn
lambda expression to each member of the Array or Page
collection, and returns the results of each application in a new collection of
the same type. If a Page is passed, its cursor is preserved in the result.
Map applies the
lambda expression concurrently to each element of the
collection. Side-effects, such as writes, do not affect evaluation of other
lambda applications. The order of possible refs being generated within the
lambda are non-deterministic.