Collections that are never empty.

NonEmptyList<int> numbers = [3, 1, 2];
var names = NonEmptyList.Of("Ada", "Grace", "Linus");

int first = numbers.Head;
int last  = numbers.Last;

if (numbers is [var head, .. var rest])
{
    Console.WriteLine($"{head} then {rest.Count} more");
}

List<int> tail = numbers[1..];

NonEmptyList<string> sorted =
    from n in names
    orderby n.Length, n
    select n.ToUpperInvariant();

NonEmptyList<int> doubled = numbers.Select(n => n * 2);  // not IEnumerable<int>
IEnumerable<int> big = numbers.Where(n => n > 10);       // may be empty, so not promoted

int    max     = numbers.Max();      // int, not int?
double average = numbers.Average();
int    sum     = numbers.Sum();      // no seed needed
double median  = numbers.Median();

Money total = invoice.Lines.Select(l => l.Amount).Sum();  // any type with a + operator

ConstrainedList<Email, And<AtLeast<Two>, Distinct>> recipients = [a, b, c];
ConstrainedList<Candidate, AtLeast<Two>> finalists = ranked.Constrain<Candidate, AtLeast<Two>>();

Candidate winner   = finalists.Head;
Candidate runnerUp = finalists.Second();   // compiles only for AtLeast<Two> and up

Validation<User, ValidationError> user = Validation.Combine(
    Validation.NotBlank(input.Name,  new ValidationError("required", "Name is required", "name")),
    Validation.NotBlank(input.Email, new ValidationError("required", "Email is required", "email")),
    Validation.Ensure(input.Age, a => a >= 18, new ValidationError("range", "Must be an adult", "age")),
    (name, email, age) => new User(name, email, age));

return user.ToResult();   // 200 with the user, or 400 validation problem details