letFn<A, B> function
Implementation
MergedMap<String, A, B> letFn<A, B>(
String name, B Function(Map<String, A> fa) f, Map<String, A> fa) {
final addedValue = f(fa);
final addedMap = {name: addedValue};
return MergedMap<String, A, B>(fa, addedMap);
}