List.cons x l equals y::l if x is Some y and l otherwise.

List.flatten l is the list of all the ys such that l contains Some y (in the same order).

List.map f [a1;...;an] is the list Some [b1;...;bn] if for all i, there is a bi such that f ai is Some bi; it is None if, for at least one i, f ai is None.