C#: Extension to Flatten Tree iterating through List of Tree's

gistfile1.cs

public static IEnumerable<T> Flatten<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> childrenSelector)
        {
            foreach (var item in source)
            {
                yield return item;
                foreach (var child in childrenSelector(item).Flatten(childrenSelector))
                {
                    yield return child;
                }
            }
        }

var _COA = GetObject<Account_ChartOfAccounts>(API.Get(API.Accounting.Actions.ChartOfAccounts, AccountID.ToString()), DataFormat.json, false, false, 100);
            Dictionary<long, string> list = new Dictionary<long, string>();
            foreach (var coa in _COA.Flatten(x => x.SubAccounts))
            {
                list.Add(coa.ID, coa.Name);
            }