Build.MapSourcesuperpose a b is b augmented with bindings of a that are not in b.
Return a map of (k, v) bindings such that:
v = f init @@ f v1 @@ fv2 @@ ... @@ f vn where v1, v2, ... vn are the values associated to k in the input list, in the order in which they appear. This is essentially a more efficient version of:
of_list_multi l |> map ~f:(List.fold_left ~init ~f) is_subset t ~of_ ~f is true iff all keys in t are in of_ and f is true for all keys that are in both.